Use a file scanner such as "fileseek" (on a PC) to scan for the definition of the item being reported in your error messages and then copy the code where you find it to custom.scss.
So if you scan for "$font-family-promo" you will find that it is defined in Versla in/templates/rt_versla/scss/configuration/ _typography.scss
$font-family-promo: "sourcesanspro", "Helvetica", "Tahoma", "Geneva", "Arial", sans-serif
So you add that to your custom.scss after the import of "depenedencies" and before you import any other particle scss.
Obviously you will need to change the fonts used too as it is unlikely that your recipient template is using the same fonts.
You will find that %g-button is define in /templates/rt_versla/scss/versla/styles/_buttons.scss so for that one you would need to copy the whole scss file to /templates/g5_helium/custom/scss and then import it in your scss.