Set ui global body color on update.

[dylanfprice]

I ran into a bug with my recently merged PR #1711. When switching the theme using the theme command, the global body color is not updated.

I didn't notice this until I switched to a dark theme and also changed my terminal background color to match. The background color of alot remained what it was in the light theme.

The issue is that the global body attribute is only set in the UI.__init__ constructor. I fixed it by setting the global body attribute again in the UI.update method.

Testing:
I did basic manual testing that the theme command now changes the background color. Doesn't look like there are any unit tests for the UI class so I didn't attempt to add any.

[pazz]

does the above commit fix the issue for you?

[dylanfprice]

@pazz yes

Here is where the global body colors are set in UI.__init__. I just did the same in UI.update.

[pazz]

Thanks for the quick fix