Change color of Solarized theme cursor

From #33

Oh yeh I noticed the cursor for Solarized Dark doesn't show up well when you switch to light mode (like I do in Vim). I made a theme with a magenta cursor and slight different opacity and now I can get rid of line and column highlights again :)

https://gist.githubusercontent.com/Riprock/683649ae7c946323331aaec812bb8dbc/raw/3ff3a7b855c0a2aefd1028addae436e9ced21aa9/solarised-dark-magenta-cursor.js

I'm not sure how common using both light and dark is, might be worth updating the built in one.

Compare the linked Solarized-dark-magenta-cursor gist with the repository's built-in Solarized theme; no file or test is named, so first locate that theme implementation. Done means the built-in cursor remains clearly visible when switching between light and dark modes without requiring line or column highlights.

Written by the indexing model from the issue text.