The colors feature set specifically strikes me as extremely difficult to implement. Using "your" color palette would require a standard for color palette usage names which would have to be meaningful across multiple different design ideologies (HIG, Material, etc.). And then on top of that it would have to prevent contrast issues? That is significantly more difficult to do in any sane way than that feature list suggests.
It's amazing what lengths people go to in order to get custom colors and fonts: they'll decompile apks and upload new ones with a custom colors.xml; they'll root their phone and change the colors in the OS; or they'll install a binary of unknown providence where someone else did this.
There are things you can do to make it easier. You can provide settings for specific elements (for example, HN has "topcolor" in user settings). If you're doing a web app, keep the markup simple so that one can use per-domain user stylesheets (try inspecting the markup of a HN page). If you're doing a Windows app, use native controls so they can be themed by the OS. But these decisions constrain what product and graphics designers can do, so you need buy-in from the other parts of your team.
A long time ago, I worked on a consulting engagement where the customer representative struggled to get direction from their higher-ups and made us implement a automatic color palette based on a company logo the uploaded. We easily extracted the main colors via imagemagick and then created a color palette from those by driving me colors with different hues etc. It was frustrating to not move forward on the actual business processes that the app was supposed to fulfill, but so much fun.
Getting such a library right is hard. Semantics and usage of colors in a palette are hard to generalize and there's almost always the odd extra colors that are needed to highlight something domain specific in a custom control. Also, some themes rely on gradients and mutiple subtle variations of hues and saturation to highlight boundaries, imply an ordering or prioritization, etc...
If you want to see a customization nightmare, look at the theming options in blender: the devs diligently expose every color used by the UI there. The list is massive.
If someone gets all of this right in a generic library, I'll tip my hat to them.
