Well it just add the "remote" class on HeaderBar : https://gitlab.gnome.org/World/Fragments/-/blob/fragments-v2/src/ui/window.rs#L142
imp.headerbar.add_css_class("remote");
then the styles : https://gitlab.gnome.org/World/Fragments/-/blob/fragments-v2/data/gtk/style.css
set it to a background color :
.remote{
background-color: #613583;
color: white;
}
Nothing spectacular, really :)
Any widgets could be styled, HeaderBar is a widget, so it could too ;)
Even though I really like Transmission, I recently switched to Fragments, due to the nice integration into GNOME. But it really is minimalist regarding it's settings.
Takes approx 7 seconds to launch on my Dell XPS i7.
I guess because of https://gitlab.gnome.org/World/Fragments/issues/34
Apart from this I really like it.