As of AMP 1.9.9.6, you can now easily customise AMPs appearance with custom themes. These are simply CSS files with extra styles applied over the top of AMPs defaults. These files are preserved between updates and can be switched on-demand, requiring just a page refresh.

Creating Theme Files

Theme files are simply CSS files with custom styling rules. You can write these same as any other styles. You can use your browsers developer tools to experiment and then save your changes as a new file.

See the w3c for an excellent beginners guide to CSS.

If you wish to share your themes you're welcome to do so, the only rule is they must not remove or obscure any CubeCoders/AMP branding. If you wish to do so the best way to do this is to submit a pull request with your theme in from the Theme directory of the CubeCoders/AMP repo (https://github.com/CubeCoders/AMP/tree/master/Themes)

Installing Theme Files

See: Using AMP Themes