I didn't make any editions at Userchrome.css and I didn't find any way to export the settings I made, but I will keep looking to do the same with Firefox, for now you can replicate these settings through this step by step on Floor Browser.

Set-up Guide

Step 0 - Install Floorp Browser

Floorp Browser page

Step 1 - Install addons and resources ⬇️:

Sidebery to sidebar
Tabliss to "new tab" page
DM Sans to font

Step 2 - Configure Floorp Browser and extensions:

Navigate to Floorp Settings ⚙️
- Design about:preferences#design -> Browser appearance -> Floorp Fluerial UI
- Design about:preferences#design -> Tab Bar Style -> Horizontal Tab Bar
- Design about:preferences#design -> Tab Bar Style -> Collapse Tree Style Tab
- Design about:preferences#design -> Tab Bar -> Optimise browser for Vertical Tab Bar
- Design about:preferences#design -> Other Preferences -> Round the corners of pages
- (OPTIONAL) Design about:preferences#design -> Bookmarks Toolbar -> Hide the Bookmarks Toolbar unless hovering over the navigation bar
Navigate to Sidebery Settings ⚙️:
- Sidebery Settings -> Help -> Import addon data -> Import the settings I left in the repo

Step 3 (Last Step) - Customize Themes 🎨:

Copy the contents of the userChrome.css in this repo
Open the userChrome.css file 📁:
- Floorp options -> Help -> More troubleshooting information -> Under 'Profile Directory' click on 'Open Directory'
- Navigate to the chrome folder and open the userChrome.css file
Paste the contents of the file
Restart Floorp

Bonus: Other things

If you want to use SplitView along with Sidebery, just add as a shortcut:
- about:preferences#cSK -> Split View Actions -> Add Shortcut

That's all

Enjoy your new browser style!