theme: Specify the weight, not the font, for header bar titles

theme: Specify the weight, not the font, for header bar titles

Using `font: Bold` will override the font with whatever the system
resolves a `Bold` font name to, including the font's own default size.

We want the font weight to be bold, but we want the default font name
and size to come from the GtkSettings:font-name property, like we do for
the rest of the system.