improve fresh install experience

[saxamaphone69]

custom css is enabled by default

[saxamaphone69]

• It looks bad because by default, the custom board list is the current board's 4 different links, and ss21 separates with padding, not margins (to increase clickable size)
• The notification is probably the most jarring thing, but is dismissible.
• OpenSettings isn't called on a fresh 4chan X install, so sidedish doesn't remove the : between setting names and descriptions, though rarely is someone running a fresh 4chan X, and as such fixes itself once dismissed the first time.
• sidedish removes all <style> elements, including the Custom CSS, if a user has it enabled.