Add Black UI support to termux-dialog

[brunelli]

The use-black-ui config toggle has no effect in termux-dialog, even though it works in the Termux's built-in dialogs, as per @rozPierog's commits in termux/termux-app#1293.

[cgarz]

Yea it can be quite jarring on dark themes. It should probably also apply to the widgets as well (termux/termux-widget#10). Then my eyes might finally be safe laughing