Add Themes.

Customization.md

@@ -24,3 +24,7 @@ WISH_THEME=rgb_gradient
 ## Plugins
 
 [Check out the list of plugins here](/Plugins)
+
+## Themes
+
+[Check out the list of themes here](/Themes)

Themes.md

@@ -0,0 +1,16 @@
+# Themes
+
++ plain
++ rgb\_gradient
+
+
+## Creating Themes
+
+Themes need to define the following variables.
+
++ `WISH_DEFAULT_FG`
++ `WISH_DEFAULT_BG`
+
+Plugins will fallback to these variables if they are not themed.
+
+Themes also need to define variables for plugins. For each plugin named `abc`, the theme needs to define `WISH_ABC_BG` and `WISH_ABC_FG`. Themes can optionally iterate over `WISH_PLUGINS` array to ensure to theme all elements. Check existing [themes](https://gitlab.com/ceda_ei/wish/tree/master/themes).