One of the most common uses for PowerApps in our environment is to just make customized and better-looking forms for SharePoint lists. Styling those forms consistently, though, is tedious. I know that some "pros" suggest creating a screen with a set of objects or controls and use those for controlling the styles of the form. For example, create a label called "LabelStyle", set the text color to what you want to use in the form, then set the Color property for the form's labels to "LabelStyle.Color". This works, but, frankly, it's a pain in the neck. There should be a simpler option.
What I'm thinking is a "Theme" or "Style" feature that would let the designer specify properties like text size, color, font, etc. that would then become the defaults for the entire App. They'd still be able to customize these properties on individual cards, controls or objects, but the defaults would be defined in the Theme or Style.
We have planned several enhancements related to theming and styles. Although this may not be 1-to-1 with CSS files, we are working on making this easier for you. We are in agreement that this is needed to expedite app building.
Thank you for your patience as we work toward enhanced theming and styles.