The key is to open Mendix Atlas UI KIT in Figma (you can find the UI Kit on Figma pages, just google the name). In the UI kit, you create a new page and start designing. You use assets from the kit so your design fits Atlas in Mendix. You could design something totally original but you will have harder time to fit it to Mendix (but if you are advanced with SCSS, it is doable anyways).
After the design is done, it is best to create UI Resources Package (UIRP) in your Mendix app (see the Mendix docs). That is just a special module where your "theme" will be stored. In the UIRP you create your page and popup layouts and start designing. You essentially change how Mendix widgets look by using SCSS.
Hi Bruna,
It's not just copy and paste. I would suggest you make a design system.
https://academy.mendix.com/link/paths/135/Implement-a-Design-System
Or for smaller changes:
https://academy.mendix.com/link/paths/123/Style-your-App-with-Sass
All details are mentioned in those modules. I would make a translation between the Figma variables and the Mendix ones.
Go Make It