Selecting TextArea theme on light/dark mode toggle

[davetapley]

It feels weird that TextArea doesn't change colors on light/dark mode toggle.

I could implement it by e.g. overriding action_toggle_dark and sending a message to set a corresponding text_area.theme.

... but it would be nice if this was handled automatically with 'sensible defaults' and/or by specifying a pair of themes.

Thoughts?

See also:

• Allow for basic "from CSS" theming of a TextArea #3468