I got asked today:

How do I make the sidebar in evolution as narrow as the one in thunderbird, so I can see all my mail folders ? Doesn’t GTK+’s awesome CSS theming make that sort of thing very easy ?

Well, I was hopeful that it would, but I figured that I better try before saying yes. Here is what I came up with:

First, create a local directory to hold our theme modification:

mkdir ~/.local/share/themes/Adwaita/gtk-3.0

Next, copy the gtk.css file from the system theme:

cp /usr/share/themes/Adwaita/gtk-3.0/gtk.css ~/.local/share/themes/Adwaita/gtk-3.0

Well, that didn’t go as expected:

What went wrong ? It turns out that the Adwaita css file is pretty minimal:

@import url("resource:///org/gnome/adwaita/gtk-main.css");

All the secret sauce is wrapped up in a resource bundle in the same directory. GTK+ automatically looks for such a bundle in the same directory as the css file when loading themes. With this knowledge, making my local copy of Adwaita work is a simple as linking the resources into the right place:

ln -s /usr/share/themes/Adwaita/gtk-3.0/gtk.gresource ~/.local/share/themes/Adwaita/gtk-3.0

The last step is to add a little bit of css to gtk.css:

EMailSidebar.view { font-size: 5px; } @import url("resource:///org/gnome/adwaita/gtk-main.css");

And we reach the desired end result:

I know what you are going to ask:

How did I come up with the selector EMailSidebar.view that matches the evolution sidebar ?

The answer is, I used gtkparasite, which is a pretty useful debugging tool for this sort of problem.

And because it is so useful, we are planning to include something very similar to it in GTK+ itself soon. You can follow this bug to track the progress.