I’ve only played the first few minutes of the first Half Life (I know, it’s on my list™). I had to turn off texture filtering immediately; the game looks terrible otherwise. Question: Why did games of this era (Morrowind also comes to mind) look this way, i.e. blurry?
Use gsettings:
gsettings set org.gnome.desktop.interface gtk-theme 'Adwaita-dark' gsettings set org.gnome.desktop.interface color-scheme 'prefer-dark'