Settings style tab cosmetics

Just some minor visual things in the settinhs app "style" tab.

1) when the preview is collapsed and "theme" is "custom" and "background" is "color" or "photo", there is a thicker border in the preview covering the edges of the corner keys

2) with the preview expanded the edges of the corner keys are cut

3) no need to indent "keyboard background" and "background (dark mode)" options