Customizer does not match live site

I am using this theme to operate an intranet blog for my organisation, so unfortunately I cannot provide links to show the issue I am having (as you have to be logged on to the organisation's network to view it). The customizer shows the pages as I want them, but in the "live environment" some of the CSS seems to get lost. So for instance, the blog entries and comments and comment boxes all have rounded corners in the Customizer, but square corners on live (not that this is particularly important). However, when I have used the Columns layout in Gutenberg editor, everything works fine in Customizer and content is displayed in columns, but in live it appears as one column only. The site is running on a Worpress Multisite Platform at version 5.3.2