Toolbar extension button hard to see on dark themes

[eishiya]

The button for toolbar extensions is a dark grey colour, which makes it hard to see when using the Tiled Fusion theme is a dark grey or other darkish colour:

If the Native theme inherits its colour from the OS on some OSes (it doesn't on Windows 10), this issue affects it too.

It would be nice if these buttons changed their colour based on the theme colours, if that's possible. The QSS selector for them is apparently QToolBarExtension, but since Tiled doesn't use QSS, I doubt that information is useful.

Unsurprisingly, other icons in Tiled suffer from this issue as well, such as the layer locks, the "Dynamically wrap tiles" button, and the Animation Editor button, however, I suspect that those, being set by Tiled rather than Qt, are less likely to have some built-in way to recolour them. Some of them are raster images, which are presumably even harder to recolour at runtime, and would probably need light/dark alternates.