Result:

To do so:

1) Install TST extension

Firslty, you need to install "Tree Style Tabs" extension from here:
https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search

2) Enable following flags

Go to

about:config

and set to True following:

toolkit.legacyUserProfileCustomizations.stylesheets

3) Apply css file

Download chrome folder from this repository and put it in your profile folder
To find out its location go to

Menu -> Help -> More troubleshooting information -> Profile Directory

For flatpack users, profile folder located here:

~/.var/app/org.mozilla.firefox/.mozilla/firefox/[profile name]

4) Some minor tweaks

You could consider move TST icon to left side of toolbar, as by default - it hidden in "Extenions" icon. To do so:
Click on "Extenions" icon -> Settings icon near TST extension -> Check "Pin to toolbar"
Then, click on Firefox menu icon -> Mote tools -> Customize toolbar... -> Move TST icon to the place you think is best for you. Also, to make TST look more like Firefox native screen: Right click on its icon -> Appearance -> Theme -> Photon

5) Restart Firefox

After restart of firefox new theme would be applied