To load your custom theme, please navigate to the website settings on the GSA dashboard and select the custom theme. This will add a theme-custom class to the body element.

This will activate the styles from resources/assets/sass/v2/layout/themes/custom/index.scss.

Make your first SCSS modification

When you activated the custom theme, please open resources/assets/sass/v2/layout/themes/custom/index.scss in your favorite IDE / code editor.

In there you will find:

&.theme-custom {
    //add your custom SCSS styling code here
}

Change it to :

&.theme-custom {
    //add your custom SCSS styling code here
    h1 {
        background:#f00;
    }
}

Now run npm run development if you don't have npm run watch running. Give it a couple seconds to run. When the assets are compiled, reload the page and see your first customisation!