If you are using many colors, I don't think using this is a good idea : the cache will be overlapping all the time
That is why this is separate function from DarkTheme(Color)... You are supposed to use it only where there is a chance of speedup. Actually, the only proper use in the whole uppsrc is in the RichText, where it is used to convert the text color. Usually you do not have that many text colors...
All that said, it is entirely possible that this could be improved somehow. But I just wanted tiny simple cache for RichText...