From your Shopify admin, go to Online Store > Themes.
Find the theme that you want to edit and click Customize.
In the left-hand toolbar, click Theme settings.
Click the For developer.
Check/Uncheck to Enable/disable load your custom CSS.
If you have knowledge about css/html, you can make any change by yourself
in “Assets/custom-css.scss.liquid” file. By default, the theme does not load
"Assets/custom-css.liquid", you have to go to Customize Page > Theme Settings,
scroll down to "Developer" and click it, tick "Load your custom css"
to the theme load "Assets/custom-css.scss.liquid".
Please take care when you make any changes in the CSS and JS files.
Because it can make your site broken or conflict with some function,
make it can not work right. So, you should backup your site first.
One more, You should modify code in “Assets/custom-css.scss.liquid” file,
we don’t ensure if you change in the other files.
Please keep in touch with us if you have any discussions via
our support center https://boostheme.zendesk.com/hc/en-us/requests/new.