I think it would be awesome to be able to create custom themes in PowerApps. In PowerBi, it's possible to create a custom theme by simply creating a color pallete and using that for your custom theme. I think it would be awesome if you could create custom themes in a similar way. Then, everytime you create a new app, as long as you can have that color palette saved, you can just create the custom theme. I feel like this would make the design process a little quicker and easier.