Great initiative! Here are a couple of areas I believe are important to make it easy to develop themes.
- Use a documented naming convention for CSS classes
- Fast development workflow to update themes
- Control and customization over Discourse features
Whether you go for SUIT CSS naming conventions or something else is a topic for another day, but with a documented naming convention people would find it easier to recognise classes, which to use and which to avoid. Avoiding ids in the default Discourse CSS would be nice as well as removing unused classes in the markup.
If it is possible to poll, pull themes via git and update themes automatically (or something smarter?) it would be really great. Also, is overwriting or extending handlebars templates something that is encouraged? If so, themes should have an easy way to do so.
If it's possible to decide which Discourse features you want and not, it would be even easier to theme. For instance, some won't want the new (and very useful) sidebar on topics or the sticky header . Instead of hacking these features away it would be nice with a checkbox to simply disable those features. We should also make sure that all colours in use actually have a corresponding setting in the color-section of the admin panel.