[ Themes ] Allow to disable zoom brightness variation, via Color Palette

### Is there an existing issue for this?

- [x] I have searched the existing issues and checked the recent builds/commits

### What problem are you experiencing?

I was trying to create a "medium grey" theme, with dark accents, like this:

https://extensions.blender.org/themes/theme-xsi/


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So, I edited the grid image, like this:

` "BACKGROUND_IMAGE": "data:image/png;base64,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",`

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But there is too much brightness variation, when we change zoom.

This seems like a smooth transition, but when we "zoom in" fast, it's like staring into a flashlight !

It's not usable.












### When does this problem occur?

When creating new themes.

### How often do you encounter this problem?

Multiple times per day

### How much does this problem affect your workflow?

Causes moderate inconvenience

### Current workarounds

_No response_

### Ideas for solutions (Optional)

Add a setting in the Color Palette to remove or decrease brightness variation, when we change zoom.

### Additional context

I'm trying to get better "eye saving" themes.

- https://github.com/Comfy-Org/ComfyUI_frontend/issues/5531
- https://github.com/Comfy-Org/ComfyUI_frontend/issues/5557

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-5558-Themes-Allow-to-disable-zoom-brightness-variation-via-Color-Palette-26e6d73d3650817a897cdc7384cec86a) by [Unito](https://www.unito.io)

The issue does not name files or tests. Start by locating the Color Palette and theme handling, then trace the zoom-related brightness variation to determine where a disable or reduction setting belongs. Done means a theme can control this variation and zooming no longer causes unwanted brightness changes.

Written by the indexing model from the issue text.