Light mode polish

- [x] Arrow on right side of theme menu item even though menu opens on left (https://github.com/oxidecomputer/console/pull/3061#issuecomment-3947282318) - [x] Light badge too low contrast (https://github.com/oxidecomputer/console/pull/3061#issuecomment-3947337296) - [x] Purple/blue preview banner too saturated (https://github.com/oxidecomputer/console/pull/3061#issuecomment-3947405043) - Also true of blue message box, e.g., in firewall rule form. Same color, I think - [x] Text in regular table cells in light mode is too light (https://github.com/oxidecomputer/console/pull/3061#issuecomment-3947405043) - [ ] https://github.com/oxidecomputer/console/issues/3086 (not really light mode specific but we should fix it together with it) - [x] Serial console doesn't really work in light mode — needs a border at least to distinguish the terminal area. But maybe it's simplest to just make it always black? Eh I'm sure we can make light mode work. Though it currently doesn't change theme until refresh when you switch the theme. If we can't easily fix that, we should just keep it white text on black. - [x] Badges in combobox