Feature: Markdown style based on fully customizable CSS

[Harald Hanöfner]

Currently in "Settings/Theme/Markdown Style", I can create a new theme and I can configure some colors and other styling. 

It would be great to allow optionally to modify the based CSS directly or to use a fully self-maintained CSS style, which would give us the option to style much more individually. E.g. table alignment, indentations, etc. 

[Rohit Sharma]

Hey, App doesn't render content using CSS, rather it uses a text view library Markwon.