usage: add wallpaper & colors

src/content/docs/en/i-i/02.usage.md

@@ -63,6 +63,11 @@ _Available on the left sidebar_
   - Include a number to go to that page
   - <sub><sub><sub>Why not just use the web interface, you ask? filters are disabled by default in the API</sub></sub></sub>
 
+# Wallpaper & colors
+- To pick a wallpaper, press `Ctrl`+`Super`+`T` and select an image. Widgets, Gtk apps (after restarting them), and Qt apps (see [how to set it up properly](https://end-4.github.io/dots-hyprland-wiki/en/i-i/01setup/#not-so-optional-stuff)) will be themed with Material colors.
+- If you'd like more vibrant or muted colors, you can change the Material color palette
+  - Press `Super`+`,` to open the color palette panel. Click the little arrow at the bottom-right corner of it for options.
+  - Choose the desired palette, like Tonal Spot (the normal one), Neutral (more muted colors), or Vibrant...
 
 # Workspace Groups