Hi Yusuf Uestuener
You can change the styling in the Properties Appearance tab but the best practice is to create a custom file and import it into the custom main class.
There is a learning path to style your App with Sass. hope this will helpful
https://academy.mendix.com/link/paths/123/Style-your-App-with-Sass
As far as I know, Mendix has no theme-library to choose from. Although there are some template applications to cherrypick from.
this may be a bit advanced, but this is how you can set up an enterprise level design system:
https://medium.com/mendix/setting-up-your-modular-design-system-in-mendix-9-6-eed2d0063411