The “Category:” word is not applied by the theme, but by WordPress itself. However, the theme conveniently provides an option to remove this at Appearance → Customize → Theme Options → Others →“Archive page title prefix”.
Sorry to bother you again about this issue, but I follow the steps: Appearance → Customize → Theme Options → Others →, to get rid of the /Category:/ and it was working fine, but is back again and now is not available an option to remove it. May you know why? Thanks.