I know the title be a little misleading. PowerApps doesn’t support CSS as of now (maybe sometime in future), but you can create a CSS light for simple tings while we await the custom Themes for PowerApps.
I find it extremely annoying when creating a large app and you need to customize visuals. If you make a change to one “screen” you have to make the same change over and over again.
In this example I declared a few variables in the beginning of the PowerApp, and one of these is the button color “RED”.
As you can see, my button color will be read when I choose to give them this color.
So at some point I decide that I want the color to be the same as my heading, and all I have to change is the color code in the app “onload”.
The result is matching buttons without having to manually change each color setting!:)
NB! after changes in the “onload”, you have to refresh the browser to see the effect.
CRM Keeper 
You don’t have to refresh the page, just go to App.OnStart and there’s 3 dots there in table view. Click those, “Run OnStart” and it will run the OnStart without you having to refresh to do it.
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Good point, but when configuing the app, it’s not always it responds to this. That is why a refresh of the screen can sometimes be a good idea 🙂
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