simple doc about theming

docs/theming.md

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+Theming Riot
+============
+
+Themes are a very basic way of providing simple alternative look & feels to the
+riot-web app via CSS & custom imagery.
+
+They are *NOT* co be confused with 'skins', which describe apps which sit on top
+of matrix-react-sdk - e.g. in theory Riot itself is a react-sdk skin.
+As of Jan 2017, skins are not fully supported; riot is the only available skin.
+
+To define a theme for Riot:
+
+ 1. Pick a name, e.g. `teal`. at time of writing we have `light` and `dark`.
+ 2. Fork `src/skins/vector/css/themes/dark.scss` to be teal.scss
+ 3. Fork `src/skins/vector/css/themes/_base.scss` to be _teal.scss
+ 4. Override variables in _teal.css as desired. You may wish to delete ones
+    which don't differ from _base.scss, to make it clear which are being
+    overridden. If every single colour is being changed (as per _dark.scss)
+    then you might as well keep them all.
+ 5. Add the theme to the list of entrypoints in webpack.config.js
+ 6. Add the theme to the list of themes in matrix-react-sdk's UserSettings.js
+ 7. Sit back and admire your handywork.
+
+In future, the assets for a theme will probably be gathered together into a
+single directory tree.