Very dim pictures on musescore.org

I guess your browser is set to dark theme so the background of that picture is also dark? Is that what caused the "dimness" you referred to?

Not dim to me. But it is a transparent png, and those don't show well in Chrome.
("don't show well" is an euphemisim for "not at all, black screen" ;-))
Unless you install an extension for that (line this), but that in turn is not available for the mobile (version (not for Android at least).
Edge has the same issue as Chrome, but no such extension for transparent PNGs (yet), as far as I can tell.
Internet Explorer works

Strange enough this inline version does work, not your link though, regardless being the very same file

don't use transparent PNGs

Not a musescore.org issue, but one of the browser used, so nothing we can do about it

Rather "invalid" than "won't fix". But this tracker doesn't have that option...

I took "won't fix" to mean "can't fix" ;-)

It probably means a little harsher, like "we acknowledge this is a problem, but we will not fix it".

Rather "it ain't broke, so won't fix" IMHO

Ah, finally, thanks. I had tried that a couple weeks ago without luck

Now I'd just need it for Chrome on Android too...