theme: Fix menu arrows

After the latest texture cache changes, loading the icon at its preferred size and scaling it via the actor size no longer works. Instead, use the icon-size property which is applied when loading the icon.

This is a case of unfortunate timing: I did notice that the arrows were ridiculously small, but as librsvg#433 (closed) was messing up all my icons, I didn't look into it.

It turns out that those icons were doubly broken, and we still have to fix the issue on our side 🤷