Can’t test myself because your theme is neither free nor from Shopify theme store.
Yes, sometimes this may not work because, say a part of the header is not part of the header section 
but this does not see to be the case this time.
You can also try “Custom CSS” under “Theme settings” – these rules are not scoped to any particular section and apply to entire site.
And yes, very rarely you may be forced to actually edit theme code for your CSS.