Add a configuration to change the preview page background

[tamlok]

Hi, shall we add a configuration to change the background color of the preview page? That is we can set the default background color to a more eyes-friendly one, such as light green, light yellow or the color of the parchment? Thanks very much!

[iamcco]

sorry for my late reply. it's a good feature to add a configuration to change the style but i don't have much time to do this, PR is welcome.

if you want to change the style immediately, you can just change the autoload/server/static/styles/markdown/markdown.css file.