Can't seem to find the correct CSS selectors for public buttons - contributions pages (Save, Cancel and Review Your Contribution)
It should be something like
div.crm-submit-buttons i { display: none; }
div.crm-submit-buttons span { /* your styling */ }
div.crm-submit-buttons input { /* your styling */ }

If you want it specific to the page you'd add something like div.crm-contribution-page-id-1 or div.crm-profile-name-volunteer_interest in front.