Tweak selection color for paperlike style

[waldyrious]

As originally mentioned in #41. Since then, ::selection became supported in the major browsers (e.g. it became unprefixed in Firefox in v62, released on 5 Sep 2018), so it should now be safe to use.

Some options:

• A lemon-lime color resembling the fluorescent yellow of highlighter markers:

  ::selection { background-color: #EF6; }

• A warmer yellow (maize), similar to Gold but not as dark:

  ::selection { background-color: #FE6; }