I think if you don’t use a 3rd party Bubble tool/plugin or someone else’s template, the preferred Bubble way is to go to the Styles tab on the left, then the Style Variables tab on top. Select the app’s font and program in its color variables. (Add more color variables as needed.) Then go to the Element Styles tab and select specifics about each kind of element (such as buttons), always using color variables. If you ever change your mind about a color or the font, you can change them in the Style variable tab and they’ll change everywhere, so long as you don’t make exceptions with individual elements. I converted my app to dark mode (no light mode) last week this way.