Say I have a base XMLUI theme `/Mirage1`, and another theme `/Mirage1-green`, and `/Mirage1-red`. How can I make it so that `Mirage1-green` and `Mirage1-red` don't have to have copies of all the XSL code and logic, but instead just have green.xsl, green.js, and green.css, and then the other theme has red.xsl, red.js and red.css?

My goal for doing this is so that I can make bug fixes and other improvements to the base theme Mirage1, and not having to repeat those fixes to the other themes too.

I'm thinking that the solution involves `Mirage1-green` theme's sitemap.xmap to import Mirage1-green/xsl/green.xsl, which then imports `../Mirage1/aspect/artifactbrowser/common.xsl` and so on, but links to code, or other guides would be much appreciated.