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. Styling the content walks through what you need to get started. We'll answer questions like: How do I make text red? How do I make content display at a certain location in the (webpage) layout? How do I decorate my webpage with...

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