Inkscape master: Panel and document tabs don't apply theme tab styles

Summary:

The new panel tab system inkscape!6945 (merged) (and the document tabs inkscape!6654 (merged) ) doesn't seem to apply tab styles from themes.

theme	before	after
breeze-dark
e17gtk-revolved
greybird

Steps to reproduce:

• open Inkscape
• change theme to a theme other than Inkscape and Default
• observe

What happened?

Tab styles are not applied to panel tabs and document tabs.

What should have happened?

Tab styles are applied to panel tabs and document tabs.

Version info

Inkscape 1.5-dev (5eccefde37, 2025-03-07, custom)

                      Compile  (Run)
    GLib version:     2.82.5
    GTK version:      4.16.12 (4.16.12)
    glibmm version:   2.82.0
    gtkmm version:    4.16.0
    libxml2 version:  2.13.6
    libxslt version:  1.1.42
    Cairo version:    1.18.2 (1.18.2)
    Pango version:    1.56.1 (1.56.1)
    HarfBuzz version: 10.4.0 (10.4.0)
    Poppler version:  25.03.0

    OS version:       Slackware 15.0 x86_64 (post 15.0 -current)