Best way to write maintainable CSS?

STOP using 100k of HTML to do 16k's job.

STOP using 500k of CSS in 12 files to do 32k in one file's job.

STOP using 2 megabytes of JavaScript in several dozen separate files to do 48k or less in one file's job.

Then, STOP using SCSS as it will then no longer serve any legitimate purpose apart from writing more code than you'd have in the finished product, to have the derpy pre-processor vomit up even more code.

If you need what SCSS "does" to maintain your CSS, you're likely using two to ten times the CSS and five to twenty times the HTML needed to do the job. It brings NOTHING to the table apart from teaching bad habits, making you write longer harder to maintain code, and a general ignorance of how any of this stuff is supposed to work.