Allow UI to adapt to smaller screen widths

As part of the GNOME Mobile apps initiative, it'd be great to have GNOME Tweaks working with smaller screens. I've made a few tweaks (heh) to make this possible. I'm specifically targeting the screens for the Librem 5 and Pinephone, which means a portrait width of 360 pixels (they magnify their desktops by 2x).

The "Fonts" screen needs a bit of resizing still, but this allows the app to at least appear on the screens. Right now the app is unusable since no widgets appear due to them all being off screen. I was motivated to make these changes since I wanted to enable Adwaita-dark on my Pinephone, which looks great now :)