Edwin; actually I don't know the the CSS language as such, but try to pick up CSS rules from other users' CSS files and then refer to online resources such as the Mozilla Developer website to see how it is used and to explore any tips and tricks. It's then back to the Firefox or Thunderbird DevTools to live debug any possibilities. Also, the inspector tool allows you to see how the Mozilla teams have actually used CSS.
developer.mozilla.org

CSS: Styling the content - Learn web development | MDN
CSS (Cascading Style Sheets) is the code that styles web content. This article walks you through a basic understanding of CSS — how it works and how to improve the look and feel of the content structure you created in the previous article.
My Computer
System One
-
- OS
- Windows 11 23H2
- Computer type
- PC/Desktop
- Manufacturer/Model
- custom
- CPU
- intel i7-8700 (non-K)
- Motherboard
- Asus Z370 TUF Gaming
- Memory
- 32Gb
- Graphics Card(s)
- On-board Intel iGPU
- Sound Card
- On-board Realtek
- Hard Drives
- Samsung_SSD_850_EVO
- PSU
- Corsair Rm850X
- Cooling
- All air