Style the .line-numbers div to be compatible w/ both themes and the GPU

The .line-numbers div has to have an opaque background because it's
sent as a texture to the GPU, and otherwise it will have isuses with
subpixel antialiasing.

However, themes style the background of the .gutter div, which was
getting obscured by the opaque background of the line numbers. This
commit adds the .gutter class to the .line-numbers div as well and
ensures it always fills the entire height of the editor.
Add a comment explaining the .editor-colors class on .lines