:lipstick: Dark mode fixes and improvements

[joshdavies14]

Resolves #46 Resolves #47 Mostly resolves #48 minus points raised in that issue

Issues fixed:

• Show alternative sponsor images if in auto dark mode
• Move theme selection to button toggles as opposed to two switches
• Fix hr dividers not showing in dark mode
• Make borders of events more distinct in dark mode

I feel like dark mode will always need constant improvements and work here, but this is a start for now.

Deploy considerations:

• Bulma has been upgraded -> bower install will need running, along with re-generating the compressed CSS files (see upgrade instructions for that).
• Once this is merged, deploy to staging and test anything there in case there are bugs before deploying to live.