A B737-800 FLT CREW CITES INADEQUATE TXWY LIGHTING/MARKINGS AT LAS AS CONTRIBUTING TO A TXWY EXCURSION.

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THE FLT WAS BOS TO LAS. I WAS THE FO. WE DEPARTED BOS ON TIME AND THE FLT TO LAS WAS UNEVENTFUL. THE WX WAS VMC THROUGHOUT THE APCH; BUT IT WAS WELL PAST DARK. THE NIGHT VISUAL APCH WAS CONDUCTED TO RWY 25L AND LNDG WENT NORMALLY. WE TURNED OFF THE RWY AT TXWY A7 AND WERE INSTRUCTED TO TAXI ON TXWY A TO TXWY A5 AND HOLD SHORT OF RWY 25R; WHICH WE DID. I ACCOMPLISHED THE AFTER LNDG CHKLIST AND THEN WE WERE CLRED TO CROSS RWY 25R AND INSTRUCTED TO CALL GND. GND INSTRUCTED US TO TAXI TO TXWY C5 AND CONTACT RAMP CTL. THE CAPT MADE A R TURN ONTO TXWY B OFF OF TXWY A5. AT THIS TIME; THE CAPT MISTOOK THE INFIELD AREA BETWEEN TXWY B5 AND TXWY B4 FOR THE TXWY C/RAMP AREA AND MADE A L TURN AT ABOUT A 45 DEG ANGLE AND TAXIED ACROSS THE UNPREPARED SURFACE. BY THE TIME I RECOGNIZED WHERE WE WERE HEADED; WE WERE ALREADY OFF THE TXWY. ANOTHER ACFT RPTED THAT A CLOUD OF DUST HAD BLOWN UP BEHIND THE ACFT. I ADMIT THAT EVEN THOUGH I WAS LOOKING AT THE UNPREPARED AREA IN THE TAXI LIGHT; IT IS VERY HARD TO DISTINGUISH THE CONCRETE FROM THE DIRT. THERE WAS A SLIGHT ROCKING OF THE ACFT WHEN WE WENT ACROSS THE AREA; BUT IT WAS NOT SIGNIFICANT. WE CROSSED THE AREA QUICKLY AND WERE ON TXWY C. WE TAXIED TO THE GATE AND WROTE UP IN THE LOG BOOK THE TAXIING EXCURSION. INSPECTION SHOWED NO DAMAGE TO THE ACFT BUT LIGHT COATING OF DUST ON THE TIRE EDGES CONFIRMED WE HAD BEEN IN THE DIRT. FIRST OF ALL; IT IS VERY DIFFICULT TO DISTINGUISH THE TXWY EDGE VISUALLY. THE COLOR OF THE UNPREPARED SURFACE IS CLOSE TO THAT OF CONCRETE. SECOND; THERE ARE ALMOST NO BLUE TXWY EDGE MARKERS. THIRD; THE LINES ARE ALMOST WASHED OUT. THIS MAY BE DUE TO DUST ON THE TXWY OR MAYBE THEY NEED TO BE REPAINTED. THAT WHOLE AREA OF THE ARPT IS ALMOST A BLACK HOLE. STILL; WE SHOULD HAVE STAYED ON THE YELLOW LINE.SUPPLEMENTAL INFO FROM ACN 702149: GND CTL INSTRUCTED ANOTHER ACFT TO 'HOLD SHORT FOR ACR X CROSSING R TO L AHEAD.' I SCANNED THE TXWYS TO THE L TO ENSURE NO CONFLICT WOULD DEVELOP WITH THAT ACFT. I THEN INITIATED THE R TURN ONTO TXWY C; MOVING R OF THE TXWY CTRLINE TO MAKE A SMOOTHER ANGULAR TRANSITION. THE INTXN APPEARED TO BE VERY LARGE AND IT WAS A VERY DARK AREA OF THE TXWY; WITH NO TXWY EDGE LIGHTS. AS I CONTINUED TO SCAN FOR OTHER TFC IN ALL DIRECTIONS; I SUDDENLY REALIZED THAT WE WERE OFF THE TXWY AND IN THE UNPREPARED DIRT SURFACE. THE SURFACE WAS VERY SIMILAR IN COLOR AND CONTRAST TO THE TXWY; AND; FORTUNATELY; VERY FIRM AND FAIRLY LEVEL WITH THE TXWY. WE HAD JUST CAUGHT THE VERY CORNER OF THE DIRT SURFACE; WITH BOTH MAIN GEAR HAVING LEFT THE TXWY. AS QUICKLY AS I REALIZED THAT WE WERE ON THE UNPREPARED SURFACE; WE WERE JUST AS QUICKLY BACK ON THE TXWY.CALLBACK CONVERSATION WITH RPTRS REVEALED THE FOLLOWING INFO: RPTRS STATED THAT THE TXWYS ARE MARKED WITH BLUE REFLECTOR POLES VICE BLUE TXWY LIGHTS AND THAT THE LIGHTING WAS VERY POOR. THE BLUE POLES ARE DIFFICULT TO DISTINGUISH AND THE ACFT'S TAXI LIGHT WAS THE ONLY SOURCE OF LIGHTING AT TIMES. THE RPTRS DID MENTION THAT THE USE OF THE LNDG LIGHTS WOULD HAVE ASSISTED; HOWEVER; NORMAL PROCS ARE TO TURN THE LNDG LIGHTS OFF AFTER EXITING THE RWY TO PREVENT BLINDING OTHER ACFT. ADDITIONALLY; THE RPTRS INDICATED THAT THE YELLOW TXWY LINES WERE FADED AND THEY ASSUMED THAT THE AREA THEY WERE TAXIING IN WAS A LARGE; OPEN TXWY AS INDICATED ON THE CHARTS.