[Cheat Sheet] Theming with new light theme is broken

Let's make sure issue is not already fixed in latest builds first.

- [x] I verified I can reproduce this issue against [latest Integration Build of Eclipse SDK](https://download.eclipse.org/eclipse/downloads/)

## Steps to reproduce

Open the Cheat Sheet View:


The Title and Buttons have the "old" light background

As soon as you focus in and out the backgound of the title "fixes itself"

When you click a button that opens a new Cheat Sheet


You get the old style back, untill you interact again
Then it switches to a mixture of both styles:


## Tested under this environment:
* OS & version: Windows 11 ( also Ubuntu but there the effect is not that prominent)
* Eclipse IDE/Platform version (as shown in *Help > About*): 2025-06

## Community

- [x] I understand reporting an issue to this OSS project does not mandate anyone to fix it. Other contributors may consider the issue, or not, at their own convenience. The most efficient way to get it fixed is that I fix it myself and contribute it back as a good quality patch to the project.