Adding theming guide. Fixes #508 (#516)

* Adding theming guide. Fixes #508

* Fix html.

* Fix html 2.

README.md

@@ -10,7 +10,8 @@
 
 <p align="center">
   <a href="https://github.com/dessalines/thumb-key" rel="noopener">
- <img width=200px height=200px src="https://raw.githubusercontent.com/dessalines/thumb-key/main/fastlane/metadata/android/en-US/images/phoneScreenshots/1.png"></a>
+    <img width=200px height=200px src="https://raw.githubusercontent.com/dessalines/thumb-key/main/fastlane/metadata/android/en-US/images/phoneScreenshots/1.png" alt="phone_screen" />
+  </a>
 
  <h3 align="center"><a href="https://github.com/dessalines/thumb-key">Thumb-Key</a></h3>
   <p align="center">
@@ -150,6 +151,16 @@ I'd be happy to accept additions to add more languages, and layouts. To start:
 - Add the new keyboard to the [KeyboardLayout.kt file](https://github.com/dessalines/thumb-key/blob/main/app/src/main/java/com/dessalines/thumbkey/utils/KeyboardLayout.kt), with a new higher index.
 - Either open a pull request, or an issue, linking your new keyboard file. I'll handle the rest of the work.
 
+## Theming guide
+
+To add a custom theme:
+
+- Use google's [Material 3 custom theme generator](https://m3.material.io/theme-builder#/custom), and save your `Theme.kt` file.
+- Copy the theme into `Color.kt` like the others.
+- Add the theme to the `ThemeColor` enum in `Types.kt`
+- Add a translation string in `strings.xml` for your theme name.
+- Add the theme to the `colorPairs` in `Theme.kt`
+
 ## Support / Donate
 
 Thumb-Key will always remain free, open-source software. We've seen many keyboards (and open-source projects in general) go unmaintained after a few years. **Recurring donations have proven to be the only way these projects can stay alive.**