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
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: