Focus styles for option controls

[matatk]

The <select>s don't have any focus indication; the others have quite weak focus indication. This should be provided by the browser but isn't (Chrome provides some, though it's not massively thick), so does need to be addressed.

(Testing this has reminded me that a separate options page is a very good idea, as it could be made easier to reach for keyboard-only users. Maybe a good idea to switch to that soon. Further: originally I was thinking the user should use the in-built browser methods for opening the options page, but given the Firefox macOS full-screen mode bug, and the fact that it would be easy to add options and help buttons to the pop-up when it's using a tree (as it'd only be a couple of tab stops to get to them), plus it's quite a lot of steps to reach the options in any browser, that too is sounding like a good idea.)