To increase the flexibility and ease by whcih PowerApps can develop for desktop and non-mobile platforms, it would be a good idea to set the default font and textbox sizes when creating forms (rahter than having to come back after the fact and change these values for each datacard one by one manually--or perhaps add a Text Size property for Form controls.
Generally improving theming is something we'll be working on over the next several months. While we are a ways from arbitrarily setting your own master font/text styles, we have recently added some improved themes that are meant to work better on the web, specifically when embedding in SharePoint, and may address some of these concerns.