See it's very slow to edit touchbar button background colors and padding
Currently I don't plan to add such things until a complete redesign of the interface is feasible (the current one gets slow enough when scrolling through long lists, which is a technical limitation with the technique I'm using). Adding more custom views would make that even worse.
At some point I will switch to a more modern approach and drop support for older macOS versions, but that may still take a while.