If you open your project in Mx Studio, you can use a designer feature (by clikcing the brush) to define a default color set of you app.
Hi Charles, the standard bootstrap classes work out of the box and can be used on your elements by adding them to the ‘class’ box in your modeler. See this page in the Mendix Docs section for more info https://docs.mendix.com/howto/front-end/styles
Custom styling can be done by adding Sass classes, see this how-to for instance to get started using koala (sass compiler) https://docs.mendix.com/howto6/setup-mendix-ui-framework-with-koala
Good luck and let us know if this is enough info to get you started!