a mood= line
What about `mood=grumpy` for me ;-P
As a comparison I looked at the theme app of my desktop (cinnamon) and its much the same as Geany, shows the internal name and description and not the filename, for the the distro provided themes the name matches the filename _name_ and _name_-dark eg `mint-y` and `mint-y-dark`, but for third party themes its like Geany, and most do not seem to distinguish if they are normal or dark.
The problem with filters is that then some colour schemes are hidden, and if random ones are included in both thats confusing and both cases are prone to bug reports.
So I'm leaning more towards sorting not filtering, and that leads back to using the filename as the simple approach. The advantage of using structured filenames is that the current alphabetical sort can be utilised to get a suitable ordering, the only PR is one renaming the files in geany-themes, no programmers needed. And random schemes will always show up, alphabetically.