Fails, dark mode changed in Textual >= 0.86

[jonas-eschle]

In Textual >= 0.86, the dark was removed and instead the "theme" is used, as for example shown here.

I am not very familiar with the internals of uproot-browser, I would have opened a PR already, but I am a bit unsure how the whole theme thing is handled

For completeness: it currently fails when trying to display a branch with no attribute "dark"

[henryiii]

New release out!