Enable dark/light theme depending on prefers-color-scheme media feature

prefers-color-scheme is a media feature which is supported by the majority of browsers. Currently, you have to toggle themes manually in the editor screen. A more optimal approach would be initially setting the color theme by reading the forementioned feature and allowing it to be overriden by the user’s manual selection.

This exact feature is actually in progress on WriteFreely now, thanks to @qwazix! Should be merged in our v0.13 release.