Bad instrument view button formatting

[jl-wynen]

In the rendered docs, the 'toggle outline visibility' button of instrument views has a bad icon:

It alternates between an exclamation mark and question mark.

[nvaytet]

I've also noticed this. This is a new thing, it used to work.
I don't know if it's some css issue (maybe something similiar to what is discussed here?) or something else.

Note that the other icons are ok (home and camera). The icon for the outline is the codepen icon. It's from the brands section. Maybe something is up with the brands-specific icons?

[nvaytet]

If the brand icons are the issue, we could consider changing things round a little:

• Use the cube icon for the outline toggle
• Use the layer-group icon for the spatial cutting tool
• And it would also be nice to harmonise the look/feel by having an icon for the RGB axes instead of a unicode character. Would something like a ruler do? Or maybe this dice although probably too similar to the cube?

Or:

• border icon for the outline toggle and the dice for the axes?