Theme change doesn't work perfectly whilst widget is open.

[samcunliffe]

Screen.Recording.2023-05-31.at.16.51.00.mov

Re-opening or doing something to the plot seems to fix.

Discussion in #138 (comment)_ and ff.

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Also maybe related: when @dstansby switches to light theme, the background of the widget is not updated. At the moment I can't reproduce this. Separate comment in: #138 (comment)

[Chris-N-K]

This should be fixed by merge #237.