The Extra Theme tag archive pages are given a title of the form “Tag: sometag”. If you’d like to remove the “Tag:” part and just leave the title as the actual name of your tag, this can be done using the following PHP code:

function remove_tag_text_from_archive_title($title) {
    return is_tag()?single_tag_title('', false):$title;
add_filter('get_the_archive_title', 'remove_tag_text_from_archive_title');

This code can be added to the end of the functions.php file. Note that it is highly advisable to add it to the functions.php file of a child theme, rather than the Extra Theme itself, to avoid it being overwritten when Extra is updated.

Please also note that changes to PHP files can cause your site to break if done incorrectly. I suggest taking a site backup before attempting this change.