1) It would be great if the Theme would have enum like properties and PowerApps would then use those as default options on the controls. 2) With this it would even better to also be able to apply an (additional) custom JSON file to the central Theme. Of course with the option to reference the Theme Enums.
1) This would result in the option to reference the Theme in your formulas like:
The best would be that you can apply multiple custom json files and are able to apply an order to those files. So you can first creat some global settings and use those settings in lower json formatting.