Install kramdown and rouge

gem install kramdown rouge

Find available rouge syntax-highlighting style

rougify help style

Preview for various rouge styles
https://spsarolkar.github.io/rouge-theme-preview/

Create syntax-highlighting file (monokai style for example)

rougify style monokai > /css/syntax.css

And copy and paste all the codes in the ‘syntax.css’ file to where you think appropriate