Hard-coded icons and axes looks bad with non-default napari themes

[dstansby]

@samcunliffe pointed out that there are napari themes other than the default dark theme: https://napari.org/stable/gallery/new_theme.html

This issue is to track:

• Checking what napari-matplotlib looks like with the non-default napari themes set (especially the light one)
• Making any fixes to improve the look with different themes

For the second point we might need to add functionality to switch the Matplotlib style sheet being used by napari-matplotlib when the napari theme is switched.

[samcunliffe]

Cheers! I was too lazy to make an issue last night (I did take this awful screenshot though). It's a bit conflated with how I think we'll end up solving #46 & #2 but it's good to have a separate issue.

This could become a subtask of #70