fix(theming): Add adjusted color versions of default background

[susnux]

• Resolves: [BITV]: Replace default background image with darker one to pass a11y contrasts on menu  #42196

Summary

The color was adjusted by @jancborchardt for better / required contrast of the app menu

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

/backport to stable28

[ShGKme]

I wonder if we should change attribution here, as it is not an original image anymore.

server/apps/theming/lib/Service/BackgroundService.php

Lines 133 to 138 in 7d8741c

	'kamil-porembinski-clouds.jpg' => [
	'attribution' => 'Clouds (Kamil Porembiński, CC BY-SA)',
	'description' => 'Background picture of white clouds on in front of a blue sky',
	'attribution_url' => 'https://www.flickr.com/photos/paszczak000/8715851521/',
	'primary_color' => self::DEFAULT_COLOR,
	],