Please adjust the brightness settings: after entering the game, bring up the console and input "brightness" to change the value from 2.0 to 3.5, then set "gl_overbright 1". If it's still not bright enough, please adjust the settings in your graphics card control panel.
Source: CS Counter-Strike Station (www.cszhan.com). For more details, refer to: http://www.cszhan.com/csrumen/20090525/2838.html