Doesn't follow GTK theme - dark theme is not supported

[husam212]

The look of the pop-up doesn't follow GTK theme used.

[hackel]

It doesn't need to follow the GTK theme necessarily, but should at least respect the cross-platform dark/light theme preference.

[Rob--W]

I welcome a dark theme, possibly automatically via https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme (overridable by a preference).