Support dark-mode image source in gallery

[pae-blue]

Bug Report

Gallery shows default source image whether there is a dark-mode source available or not.

Steps to Reproduce

1. Create article with image component and link upload light- and dark-mode version of image
2. Publish article and open it while dark-mode is enabled
3. Click on image to open gallery

Actual Results

Default image source shows up.

Expected Results

Dark-mode image source shows up.

[tpreusse]

I think the correct solution would be to always use the dark image in gallery if available.

See the cover in the following example which has a light and dark version to illustrate the reason:
https://www.republik.ch/2021/04/20/der-stellenmarkt-braucht-ein-update

• transparent png
• light version: transparent bg + black lines
• dark version: transparent bg + white lines

Now since the bg of the gallery is always black we should always use the dark version.