Settings and connect pages don't respect current theme

[Venemo]

For example, it appears light even if the dark theme is selected.

[Venemo]

I now investigated this a bit further. The cause of the issue is that the style sheet of the dark theme does not contain any styling for the controls on the settings and connection pages.

The correct solution would be to either create a dark style sheet or switch to a dark QStyle.

[Venemo]

Options for the dark QStyle:

• Use the Fusion style and set the palette to make it dark: https://gist.github.com/skyrpex/5547015 and https://gist.github.com/QuantumCD/6245215
• On Gnome, we can just use the dark variant of adwaita (done)