NOTE: This should not be worked on until T387543 is resolved.
**Steps to replicate the issue** (include links if applicable):
* Visit desktop https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_218#Thumbnail_background in dark mode
* Visit mobile - https://en.m.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_218#Thumbnail_background in dark mode
**What happens?**:
Images use a fixed background color #c8ccd1 defined in Vector (https://gerrit.wikimedia.org/g/mediawiki/skins/Vector/+/df3e7fb3efa87111fd9de4cf23cb73f606993497/resources/skins.vector.styles/media.less#19)
and in Minerva (https://gerrit.wikimedia.org/g/mediawiki/skins/MinervaNeue/+/14ce43900462a0f82053afd084b9962896c1d068/resources/skins.minerva.styles/content/images.less#131)
**What should have happened instead?**:
* The images should be identical on mobile and desktop
* Following input from DST on T387543 the images should be updated to use a fixed white background (`@background-color-base-fixed`) in both dark and light mode
* In dark mode the images should also receive a `filter: brightness(0.8);`
* Light mode should not be impacted by our changes.
**Software version** (on `Special:Version` page; skip for WMF-hosted wikis like Wikipedia):
**Other information** (browser name/version, screenshots, etc.):
== Requirement
Improve thumbnail styling for dark mode to ensure consistent appearance across desktop and mobile. Update styles to use a fixed white background and apply a brightness filter in dark mode, while preserving the current appearance in light mode.
== BDD
```
Feature: Update thumbnail styling for dark mode legibility
Scenario: Thumbnails render consistently across skins
Given I view a thumbnail image in light or dark mode
Then the thumbnail has a white background
And it looks visually consistent across Minerva and Vector skins
Scenario: Thumbnails are filtered in dark mode for legibility
Given I view a page in dark mode
When I view a thumbnail
Then the thumbnail has a brightness filter applied
Scenario: Light mode appearance remains unchanged
Given I view a page in light mode
Then the thumbnail displays as it did previously
```
== Test Steps
Test Case 1: Verify thumbnails use white background and brightness filter in dark mode (Vector)
1. Enable dark mode on desktop using Vector skin.
2. Visit: https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_218#Thumbnail_background
3. Locate a thumbnail image.
4. AC1: Confirm the background is white.
5. AC2: Confirm a brightness(0.8) filter is applied.
Test Case 2: Verify thumbnails use white background and brightness filter in dark mode (Minerva)
1. Enable dark mode on mobile (or emulate mobile view).
2. Visit: https://en.m.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_218#Thumbnail_background
3. Scroll to a thumbnail.
4. AC3: Confirm the background is white.
5. AC4: Confirm a brightness(0.8) filter is applied.
Test Case 3: Verify light mode is unaffected
1. Disable dark mode (return to light mode).
2. Visit either desktop or mobile version of the above article.
3. AC5: Confirm thumbnail styling remains unchanged from the previous behavior.
In event of rollback, it's important not to revert 588efdff805. Reverting the Vector and Minerva changes should be sufficient.