Saving a figure as image has some style issues

When saving a figure as image while using the dark theme the style poses some problems. The axe ticks and labels remain coloured light grey as stated by the style sheet, but this makes them nearly invisible in the saved image.

I would suggest to either use a non-transparent facecolor (napari theme background) or add specific colouring for savefig to the style sheet: `figure.facecolor : 262930` or `savefig.facecolor : 262930`

Another solution would be to change the colours for the axes, ticks and labels for saving, but as far as I know this can't be achieved through matplotlib without changing the colours of the displayed figure as well.

No file or test is named. Start by reproducing a dark-theme figure save and inspect the save-to-image path alongside the theme stylesheet. Done means saved images have readable axes ticks and labels without making the displayed figure's styling incorrect.

Written by the indexing model from the issue text.