A tiny Discord css-injector.

git clone https://github.com/mlvzk/discocss
cp discocss/discocss /usr/bin # or any other directory in your $PATH

Put your css in ~/.config/discocss/custom.css

Run discocss

If you want your window to be transparent, you have to change the --background- css variables to have an alpha value. You can add alpha to a hex color by appending 2 extra hex digits to it. Example from my theme (44 is the hex alpha value):

.theme-light {
  --background-primary: #f3f4f644;
  --background-primary-alt: #eeeff244;
  --background-secondary: #e5e7eb44;
  --background-secondary-alt: #d1d5db44;
  --background-tertiary: #dfe2e744;
}

If after an update your theme doesn't work anymore, close and run discocss again. Discord might be running in the background without you knowing, so if it still doesn't work, use pkill.