I have demo'ed the issue in https://en.m.wikipedia.org/wiki/Draft:Einstein%27s_thought_experiments and have attached a photo illustrating the appearance of the footnote on my iPhone. In the actual published article, it was necessary for me to render the math using HTML.

Developer notes

The math image HTML:

<img class="mwe-math-fallback-image-inline image-lazy-loaded" src="https://wikimedia.org/api/rest_v1/media/math/render/svg/3a7e98a2f04ab9437efae1963985b11956e5472e" alt="{\displaystyle 8\pi \nu ^{2}/c^{2},}" style="width: 8.676ex;height: 3.176ex;">

Math is always rendered as black, which doesn't look good when the background is also dark.