Why is the "piano" style option gone from my library? I was swapping my styles around to see if I like a different one (my favourite being the Piano) but after I swapped and tried the "Dark,"Chrome" etc, the "Piano" style has disappeared from library.

What happened and how to get it back?

I WONDERED WHAT WAS WRONG!

I've been looking at my library thinking something was off, not being able to pin it down, but that was it.
The piano style I had was replaced with wood.

Well, it's not the end of the world.

Piano style option appears to be there. In Galaxy.

It's my current view, so nothing is gone in my account.

do you use Galaxy?

No, browser only.

It looks like it is in the code, but for some reason doesn't show up for me (Firefox).

That drop down menu seems to be too short to fit part of White and entirety of Piano lines.

I think we found the common link.

Temporary fix that worked for me (using Stylus browser addon):

I can also confirm it's gone from my library (Chrome) but is still available in Galaxy.

It's working fine for me (I'm afraid to change it to see if I can select it again, though). My browser is Opera.

I just switched themes and also saw that the Piano style wasn't available afterward. But... the theme is still there, under White. Like InkPanther mentioned, it seems that the dropdown menu is too small to display all the options.

The way I fixed it was by right-clicking the menu and selecting Inspect from my browser. It displays the HTML code and you can temporarily delete one of the upper themes, which will make room for Piano to be visible again. This is something that GOG needs to fix, but hopefully this will help anyone experiencing this for now.

Nothing will happen unless someone reports it, detailed like you just did so it will be a quick fix.