Support Custom SCSS

[arjenvri]

Thank you for maintaining this great Hugo theme!

I would like to adjust some of the h1, h2 header font sizes. At this momemt this requires overwriting existing SCSS files and including existing styles.
A more clean approach would be allowing a custom SCSS to be loaded to overwrite styles.

[params]
customSCSS = ["css/custom.scss"]

Would it be possible to add this feature?