gh-36695: Dark Mode Compatibility: Improved Crystals Visibility
    
Fixes #35740

Format  `.png` images for dark mode compatibility

![image](https://github.com/sagemath/sage/assets/145871325/96a01567-
a701-476f-87cb-909f647425dc)


- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
    
URL: #36695
Reported by: cherry-1729-9090
Reviewer(s): Matthias Köppe

Author: Release Manager