For 57.0v about arc-firefox-theme

[JackCuthbert]

In case any one else stumbles across this, I made a few userChrome.css tweaks to better support the no titlebar option in Firefox 58 in this GitHub gist. Your mileage may vary, this was done up quickly in a Fedora 27 VM with gnome-shell 3.36.2. This potentially solves #136 as well.

From this:

To this:

[ArchangeGabriel]

@JackCuthbert Note that this issue you’re fixing with your userChrome.css is worked on by Mozilla and should hopefully be fixed soon. See also arc-design/arc-theme#34.

[tuananh]

I would love to use this theme with ff 57 as well.

[Vault108-zz]

Same, i love this theme and would love to use it!

[tuananh]

Or if anyone can point us to where to look at so that we can do it ourselves, that would be much appreciated.

[jadenPete]

You're going to want to start here. Then once you've enabled the Browser Toolbox you can enter it as described in the article and start modifying/testing custom CSS. You can then put your changes in chrome/userChrome.css (in your Firefox user directory) and distribute the file.

Made a new userChrome.css based theme (or maybe i should call it afew tweaks to the standard dark quantum theme). it's a more flat approach (i don't like these borders :D) compared to the original arc style but it works well for me with arc darker. if anyone wants to take a look:
https://github.com/jens-t/firefox-theme-arc-darker-flat
(in the theme readme you can also find infos and links on how to work with the userChrome.css)