If the map looks too bright, or there's no lighting at all, here's how I fixed it:
1. turn on cheats by entering "sv_cheats 1" into the developer console
2. turn off fullbright by entering "mat_fullbright 0" in the dev console, only works with cheats on
3. Go into your advanced video graphics settings and turn "High Dynamic Range" (HDR) to full
4. Restart your game to apply all changes, and make sure you have CS:Source installed

my game still crashes with an empty error window labelled "engine error" but I have no idea how to fix that part