I suggest that all error messages in all the templates should be enclosed in HTML tags <span class="error-message"> or something similar. Then customizers could cause error messages to appear in red, a more emphatic font, etc. just by editing their style sheets.
Perhaps the default style sheet could then have error messages displayed in red at least.
Currently, error messages appear in the standard small black font, and it isn't very easy for a user to find them by eye to determine what's wrong.