Added "Theming" section (#112)

README.md

@@ -112,6 +112,11 @@ bindsym XF86MonBrightnessDown exec swayosd-client --brightness -10
 - It only changes the target device for the current action that changes the volume.
 - You can list your input audio devices using `pactl list short sources`, for outputs replace `sources` with `sinks`.
 
+## Theming
+
+Since SwayOSD uses GTK, its appearance can be changed. Initially scss is used, which GTK does not support, so we need to use plain css. 
+The style conifg file is in `~/.config/swayosd/style.css` (it is not automatically generated). For reference you can check [this](https://github.com/ErikReider/SwayOSD/blob/main/data/style/style.scss) and [this](https://github.com/ErikReider/SwayOSD/issues/36).
+
 ## Brightness Control
 
 Some devices may not have permission to write `/sys/class/backlight/*/brightness`.