Hello Nuno,
In general, for styling and developing purposes it is good to follow the leanring paths around page creation and domain models, further as extra you can follow learning paths around the use of nanoflow and microlfows.
Other interesting topics can be learning paths around native(this is a bit a different kind of styling)
Hope this helps
Good luck!
It might also be usefull to take a look into the default Atlas UI design system Mendix uses. It should familiarize you with all the default building blocks, page elements and widgets you can use 'out of the box'. You can find that here: https://atlasdesignsystem.mendixcloud.com/p/dashboard
Additionally I would also suggest to create a default app, place some page widgets on a page and inspect the html through through the browser in order to get to know the basic HTML structure the page elements of Mendix yield upon using them.
Lastly you should be able to play around with the default design properties Mendix offers. I see a lot of projects were a lot of custom classess are implemented for the same things that can be done with the design properties. The same goes for default Bootstrap classess (Version 3.3.4 if I'm correct, on top of which the Atlas UI is built - https://bootstrapclasses.com/bootstrap-3-css-classes-index/)