[backport v2.15.1] Custom branding color is not applied consistently

[rancher-ui-project-bot]

This is a backport issue for #16905, automatically created via GitHub Actions workflow initiated by @aalves08

Original issue body:

JIRA: SURE-11425

Setup

• Rancher version: 2.13+
• Rancher UI Extensions: None
• Browser type & version: any

Describe the bug

Custom branding is not applied to the side nav and top nav.
Please refer to the attached video for details.
Custom color should be applied everywhere

To Reproduce

I've reproduced it on a prime instance, haven't tried on community
Global settings -> Branding -> Primary Color-> Use custom color -> Select a color -> Apply

Result
Custom color is applied to most areas, but side nav menus and top nav still have prime colors

Expected Result

Custom color should be applied everywhere

Screenshots

Screen.Recording.2026-03-10.at.4.39.18.PM.mov

Additional context