Hi there,

Ideally, any template customization can be done through the custom.css file. You may create the custom.css file in this folder.
/voyager/templates/voyager/assets/css/custom.css.

Anyhow, since you've created custom.scss file, you'll need to import it in the template.scss file for it to be compiled as css.
/voyager/templates/voyager/assets/css/scss/template.scss

However, please do understand that this template.scss is a core file. Any changes within this file will get overridden when updating this template in the future.

Hope this helps.
Thanks.