There has been a known brightness issue in Dota 2. You can’t adjust/increase the brightness level in-game, the slider doesn’t work even after you apply the changes, the game gets too dark making it sort of unplayable. Luckily, there’s a fix available for this issue using a small console command.
Guide to Fix Brightness in Dota 2:
• Open console by pressing “~” key (read this guide if you don’t have console enabled).
• Now enter this command:
– You can switch the values from 1.6 to 2.6 depending on how bright the game you want.
• It should fix the brightness problem for you. If you try reconfigure the brightness from game settings it will become dark again.
If the problem persists, you need to change the monitor gamma/brightness from your Graphic card options (for e.g. ATI Catalyst Control Cente/Nvidia Control Panel).