I believe this would be useful for you:
- Core concepts - ExtJS 6.0 theming
will give you an overview how the underlying framework -- ExtJS -- handles theming. There you'll see a link for Sencha CMD, an important part of theming -- the building interface.
I believe it would be much easier if you based on an existing theme and then replicated it.
When you update themes from ExtJS side, you can then update them on Ext.NET.
Updating an existing them in Ext.NET should be tad easy, just copy-paste the built theme files over.
Every time you update the themes on Ext.NET sources you have to run the
to refresh the ext.axd-aware files to the theme (so the theme is bundled on Ext.NET). I suggest closing the solution and running this script then, and then re-opening and rebuilding the solution to ensure all gets in place.
As for adding a new theme to Ext.NET, you'd have to change the theme list enum, include the files in the correct resource files (that indicates which css files are to be used as base to make the embedded CSS files -- linking the resources thru the ext.axd links in ASP.NET). I believe we can provide details on this as you evolve on the themes development and gets questions.
First part to master would be the theming guide linked above.
I hope this helps for now!