add transparency information in README

Author: mlvzk

README.md

@@ -19,4 +19,20 @@ Run `discocss`
 
 # Notes
 
+## Transparency
+
+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):
+
+```css
+.theme-light {
+  --background-primary: #f3f4f644;
+  --background-primary-alt: #eeeff244;
+  --background-secondary: #e5e7eb44;
+  --background-secondary-alt: #d1d5db44;
+  --background-tertiary: #dfe2e744;
+}
+```
+
+## Persistence
+
 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`.