Use HTML tags in ‘Appearance -> Customise’ interface

Thanks for the reply. So in theory, as I have all of the theme files,
would I myself be able to make these changes myself so that HTML is
supported?

It’s generally a bad idea to edit the files of a theme or plugin that you did not develop or that you do not control updates for. Any changes will erased on any update and disabling updates or forking the theme or plugin leaves you unable to receive potential bug and security fixes for a codebase that you’re less familiar with and less able to fix yourself.

If you still intend to make the change then you need to check two places:

1. The sanitize_callback setting provided to a call to $wp_customize->add_setting(). Whichever function is being passed here is responsible for sanitizing the data before it’s saved. Note that the variable name $wp_customize could be different in your theme.
2. Wherever the value of the customizer field is being output. This could be a template file, widget or almost anything else. You would be looking for a call to get_theme_mod() for that field. If the value is being output through something like esc_html() then it’s being escaped. Since you mentioned that you can see the HTML tags this is most likely what’s happening. You would need to change or remove this function to allow HTML. Since escaping is important I would review the documentation on escaping for alternative functions that suit your needs so that you don’t leave the value completely unescaped.

Anything more would require specific knowledge of your theme so I suggest reaching out to them if this doesn’t help.