Add more details to documentation about changing color theme

[uglycoyote]

Just trying to make it clear that the .tmtheme file is the one to change, and than other plugins might get in the way of the process of trying to reload the color scheme, as neither of these things were obvious to me and it caused it to take much longer than it should have to tweak a colour.

[adzenith]

Cleaned up whitespace, removed the merge commit, and merged. Thanks for the contribution!