Modifique o arquivo mime.types adicionando o seguinte:
text/css css
Abaixo estão os conteúdos do arquivo mime.types que eu uso em um dos meus servidores Apache. Estou bastante certo de que abrange praticamente todos os tipos de arquivos que o seu (ou qualquer) servidor pode servir aos usuários. Eu sugeriria fazer a alteração no arquivo mime.types do seu VPS para incluir tudo abaixo, especialmente se não houver nada relacionado a arquivos css, imagens ou html.
# MIME type Extension application/EDI-Consent application/EDI-X12 application/EDIFACT application/activemessage application/andrew-inset ez application/applefile application/atomicmail application/batch-SMTP application/beep+xml application/cals-1840 application/commonground application/cybercash application/dca-rft application/dec-dx application/dvcs application/eshop application/http application/hyperstudio application/iges application/index application/index.cmd application/index.obj application/index.response application/index.vnd application/iotp application/ipp application/isup application/font-tdpfr application/mac-binhex40 hqx application/mac-compactpro cpt application/macwriteii application/marc application/mathematica application/mathematica-old application/msword doc application/news-message-id application/news-transmission application/ocsp-request application/ocsp-response application/octet-stream bin dms lha lzh exe class so dll application/oda oda application/parityfec application/pdf pdf application/pgp-encrypted application/pgp-keys application/pgp-signature application/pkcs10 application/pkcs7-mime application/pkcs7-signature application/pkix-cert application/pkix-crl application/pkixcmp application/postscript ai eps ps application/prs.alvestrand.titrax-sheet application/prs.cww application/prs.nprend application/qsig application/remote-printing application/riscos application/rtf application/sdp application/set-payment application/set-payment-initiation application/set-registration application/set-registration-initiation application/sgml application/sgml-open-catalog application/sieve application/slate application/smil smi smil application/timestamp-query application/timestamp-reply application/vemmi application/vnd.3M.Post-it-Notes application/vnd.FloGraphIt application/vnd.accpac.simply.aso application/vnd.accpac.simply.imp application/vnd.acucobol application/vnd.aether.imp application/vnd.anser-web-certificate-issue-initiation application/vnd.anser-web-funds-transfer-initiation application/vnd.audiograph application/vnd.businessobjects application/vnd.bmi application/vnd.canon-cpdl application/vnd.canon-lips application/vnd.claymore application/vnd.commerce-battelle application/vnd.commonspace application/vnd.comsocaller application/vnd.contact.cmsg application/vnd.cosmocaller application/vnd.cups-postscript application/vnd.cups-raster application/vnd.cups-raw application/vnd.ctc-posml application/vnd.cybank application/vnd.dna application/vnd.dpgraph application/vnd.dxr application/vnd.ecdis-update application/vnd.ecowin.chart application/vnd.ecowin.filerequest application/vnd.ecowin.fileupdate application/vnd.ecowin.series application/vnd.ecowin.seriesrequest application/vnd.ecowin.seriesupdate application/vnd.enliven application/vnd.epson.esf application/vnd.epson.msf application/vnd.epson.quickanime application/vnd.epson.salt application/vnd.epson.ssf application/vnd.ericsson.quickcall application/vnd.eudora.data application/vnd.fdf application/vnd.ffsns application/vnd.framemaker application/vnd.fsc.weblaunch application/vnd.fujitsu.oasys application/vnd.fujitsu.oasys2 application/vnd.fujitsu.oasys3 application/vnd.fujitsu.oasysgp application/vnd.fujitsu.oasysprs application/vnd.fujixerox.ddd application/vnd.fujixerox.docuworks application/vnd.fujixerox.docuworks.binder application/vnd.fut-misnet application/vnd.grafeq application/vnd.groove-account application/vnd.groove-identity-message application/vnd.groove-injector application/vnd.groove-tool-message application/vnd.groove-tool-template application/vnd.groove-vcard application/vnd.hhe.lesson-player application/vnd.hp-HPGL application/vnd.hp-PCL application/vnd.hp-PCLXL application/vnd.hp-hpid application/vnd.hp-hps application/vnd.httphone application/vnd.hzn-3d-crossword application/vnd.ibm.afplinedata application/vnd.ibm.MiniPay application/vnd.ibm.modcap application/vnd.informix-visionary application/vnd.intercon.formnet application/vnd.intertrust.digibox application/vnd.intertrust.nncp application/vnd.intu.qbo application/vnd.intu.qfx application/vnd.irepository.package+xml application/vnd.is-xpr application/vnd.japannet-directory-service application/vnd.japannet-jpnstore-wakeup application/vnd.japannet-payment-wakeup application/vnd.japannet-registration application/vnd.japannet-registration-wakeup application/vnd.japannet-setstore-wakeup application/vnd.japannet-verification application/vnd.japannet-verification-wakeup application/vnd.koan application/vnd.lotus-1-2-3 application/vnd.lotus-approach application/vnd.lotus-freelance application/vnd.lotus-notes application/vnd.lotus-organizer application/vnd.lotus-screencam application/vnd.lotus-wordpro application/vnd.mcd application/vnd.mediastation.cdkey application/vnd.meridian-slingshot application/vnd.mif mif application/vnd.minisoft-hp3000-save application/vnd.mitsubishi.misty-guard.trustweb application/vnd.mobius.daf application/vnd.mobius.dis application/vnd.mobius.msl application/vnd.mobius.plc application/vnd.mobius.txf application/vnd.motorola.flexsuite application/vnd.motorola.flexsuite.adsi application/vnd.motorola.flexsuite.fis application/vnd.motorola.flexsuite.gotap application/vnd.motorola.flexsuite.kmr application/vnd.motorola.flexsuite.ttc application/vnd.motorola.flexsuite.wem application/vnd.mozilla.xul+xml application/vnd.ms-artgalry application/vnd.ms-asf application/vnd.ms-excel xls application/vnd.ms-lrm application/vnd.ms-powerpoint ppt application/vnd.ms-project application/vnd.ms-tnef application/vnd.ms-works application/vnd.mseq application/vnd.msign application/vnd.music-niff application/vnd.musician application/vnd.netfpx application/vnd.noblenet-directory application/vnd.noblenet-sealer application/vnd.noblenet-web application/vnd.novadigm.EDM application/vnd.novadigm.EDX application/vnd.novadigm.EXT application/vnd.osa.netdeploy application/vnd.palm application/vnd.pg.format application/vnd.pg.osasli application/vnd.powerbuilder6 application/vnd.powerbuilder6-s application/vnd.powerbuilder7 application/vnd.powerbuilder7-s application/vnd.powerbuilder75 application/vnd.powerbuilder75-s application/vnd.previewsystems.box application/vnd.publishare-delta-tree application/vnd.pvi.ptid1 application/vnd.pwg-xhtml-print+xml application/vnd.rapid application/vnd.s3sms application/vnd.seemail application/vnd.shana.informed.formdata application/vnd.shana.informed.formtemplate application/vnd.shana.informed.interchange application/vnd.shana.informed.package application/vnd.sss-cod application/vnd.sss-dtf application/vnd.sss-ntf application/vnd.street-stream application/vnd.svd application/vnd.swiftview-ics application/vnd.triscape.mxs application/vnd.trueapp application/vnd.truedoc application/vnd.tve-trigger application/vnd.ufdl application/vnd.uplanet.alert application/vnd.uplanet.alert-wbxml application/vnd.uplanet.bearer-choice-wbxml application/vnd.uplanet.bearer-choice application/vnd.uplanet.cacheop application/vnd.uplanet.cacheop-wbxml application/vnd.uplanet.channel application/vnd.uplanet.channel-wbxml application/vnd.uplanet.list application/vnd.uplanet.list-wbxml application/vnd.uplanet.listcmd application/vnd.uplanet.listcmd-wbxml application/vnd.uplanet.signal application/vnd.vcx application/vnd.vectorworks application/vnd.vidsoft.vidconference application/vnd.visio application/vnd.vividence.scriptfile application/vnd.wap.sic application/vnd.wap.slc application/vnd.wap.wbxml wbxml application/vnd.wap.wmlc wmlc application/vnd.wap.wmlscriptc wmlsc application/vnd.webturbo application/vnd.wrq-hp3000-labelled application/vnd.wt.stf application/vnd.xara application/vnd.xfdl application/vnd.yellowriver-custom-menu application/whoispp-query application/whoispp-response application/wita application/wordperfect5.1 application/x-bcpio bcpio application/x-cdlink vcd application/x-chess-pgn pgn application/x-compress application/x-cpio cpio application/x-csh csh application/x-director dcr dir dxr application/x-dvi dvi application/x-futuresplash spl application/x-gtar gtar application/x-gzip application/x-hdf hdf application/x-javascript js application/x-koan skp skd skt skm application/x-latex latex application/x-netcdf nc cdf application/x-sh sh application/x-shar shar application/x-shockwave-flash swf application/x-stuffit sit application/x-sv4cpio sv4cpio application/x-sv4crc sv4crc application/x-tar tar application/x-tcl tcl application/x-tex tex application/x-texinfo texinfo texi application/x-troff t tr roff application/x-troff-man man application/x-troff-me me application/x-troff-ms ms application/x-ustar ustar application/x-wais-source src application/x400-bp application/xhtml+xml xhtml xht application/xml application/xml-dtd application/xml-external-parsed-entity application/zip zip audio/32kadpcm audio/basic au snd audio/g.722.1 audio/l16 audio/midi mid midi kar audio/mp4a-latm audio/mpa-robust audio/mpeg mpga mp2 mp3 audio/parityfec audio/prs.sid audio/telephone-event audio/tone audio/vnd.cisco.nse audio/vnd.cns.anp1 audio/vnd.cns.inf1 audio/vnd.digital-winds audio/vnd.everad.plj audio/vnd.lucent.voice audio/vnd.nortel.vbk audio/vnd.nuera.ecelp4800 audio/vnd.nuera.ecelp7470 audio/vnd.nuera.ecelp9600 audio/vnd.octel.sbc audio/vnd.qcelp audio/vnd.rhetorex.32kadpcm audio/vnd.vmx.cvsd audio/x-aiff aif aiff aifc audio/x-mpegurl m3u audio/x-pn-realaudio ram rm audio/x-pn-realaudio-plugin rpm audio/x-realaudio ra audio/x-wav wav chemical/x-pdb pdb chemical/x-xyz xyz image/bmp bmp image/cgm image/g3fax image/gif gif image/ief ief image/jpeg jpeg jpg jpe image/naplps image/png png image/prs.btif image/prs.pti image/tiff tiff tif image/vnd.cns.inf2 image/vnd.djvu djvu djv image/vnd.dwg image/vnd.dxf image/vnd.fastbidsheet image/vnd.fpx image/vnd.fst image/vnd.fujixerox.edmics-mmr image/vnd.fujixerox.edmics-rlc image/vnd.mix image/vnd.net-fpx image/vnd.svf image/vnd.wap.wbmp wbmp image/vnd.xiff image/x-cmu-raster ras image/x-portable-anymap pnm image/x-portable-bitmap pbm image/x-portable-graymap pgm image/x-portable-pixmap ppm image/x-rgb rgb image/x-xbitmap xbm image/x-xpixmap xpm image/x-xwindowdump xwd message/delivery-status message/disposition-notification message/external-body message/http message/news message/partial message/rfc822 message/s-http model/iges igs iges model/mesh msh mesh silo model/vnd.dwf model/vnd.flatland.3dml model/vnd.gdl model/vnd.gs-gdl model/vnd.gtw model/vnd.mts model/vnd.vtu model/vrml wrl vrml multipart/alternative multipart/appledouble multipart/byteranges multipart/digest multipart/encrypted multipart/form-data multipart/header-set multipart/mixed multipart/parallel multipart/related multipart/report multipart/signed multipart/voice-message text/cache-manifest appcache manifest text/calendar text/css css text/directory text/enriched text/html html htm text/parityfec text/plain asc txt text/prs.lines.tag text/rfc822-headers text/richtext rtx text/rtf rtf text/sgml sgml sgm text/tab-separated-values tsv text/t140 text/uri-list text/vnd.DMClientScript text/vnd.IPTC.NITF text/vnd.IPTC.NewsML text/vnd.abc text/vnd.curl text/vnd.flatland.3dml text/vnd.fly text/vnd.fmi.flexstor text/vnd.in3d.3dml text/vnd.in3d.spot text/vnd.latex-z text/vnd.motorola.reflex text/vnd.ms-mediapackage text/vnd.wap.si text/vnd.wap.sl text/vnd.wap.wml wml text/vnd.wap.wmlscript wmls text/x-setext etx text/xml xml xsl text/xml-external-parsed-entity video/mp4v-es video/mpeg mpeg mpg mpe video/parityfec video/pointer video/quicktime qt mov video/vnd.fvt video/vnd.motorola.video video/vnd.motorola.videop video/vnd.mpegurl mxu video/vnd.mts video/vnd.nokia.interleaved-multimedia video/vnd.vivo video/x-msvideo avi video/x-sgi-movie movie x-conference/x-cooltalk ice