Refactor the style file

[longfangsong]

The initial style file is from the origin theme, which is a huge all-in-one .scss file.
This was fine to just use it when we want to keep everything of the theme as is, but during using of this theme, the users including me have found there are several more features it should have (eg. dark mode, multilingual). And to make these features come ture, I added a pile of code into the scss, and it's now become a 1k+ lines of shit pile.
We should refactor the style file with all modern features sass provides us.