Hey Paul!
If the issue goes away when you change to the default theme and come back when you switch back, it’s likely that your theme has some styles affecting the calendar widget.
I find the simplest way to figure them out is to use the inspector in your browser to see which styles are causing the issue. I don’t know if you’ve used it before, so assuming you haven’t:
You can see each of the rules applied to the calendar and enable/disable them to see which fixes the issue. Here’s an image for you of the calendar and it’s styles in the inspector. Each style rule has a checkbox you can use to toggle it off/on – by working through those you should be able to find the style conflict.

Then it’s a matter of either editing the styles of your theme or adding an overriding custom style rule under Appearance -> Customize in the dashboard.
If you need more help with this, let me know!
Cheers,
Stephen