This template (V3 coming shortly) will save you SO much time and empower your citizen developers and clients!
iAm_ManCat’s Branding Template V2
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The Power Apps Tools are unofficial and parts of them are expected to be eventually made part of the main product, however, they have helped me develop and hone the way that I work on Apps and the Theme Editor was the inspiration for my Branding Template.
The PowerApps-Tools Github repository