Crash changing theme contrast (sometimes) + unexpected results

Reported by @Moini in inbox#11420 (moved) / #5431 (closed)

Steps to reproduce:

• open Inkscape
• open Preferences
• change theme contrast in Interface>Theming

What happened?

crash or unexpected result:

#5431 (comment 2257900802)

What should have happened?

no crash

Inkscape Version and Operating System:

• Inkscape Version: Inkscape 1.5-dev (9686b905, 2024-12-13, custom)
• Operating System: Fedora Workstation
• Operating System version: 41