Some current pages in bold on menu, others not

Hi all,

I’ve just started using WordPress today on a domain I acquired, and have two separate WordPress blogs running. However, there’s something strange happening on the central menu bar. When I’m on either of the ‘home’ pages for each of the installations – i.e. the two blogs – that link is bold in the central menu. However, when I’m on any of the non-blog pages for each blog, those links are not bold in the menu. The site is: https://www.ultimaratioregum.co.uk/

If you click on ‘Academic Blog’, for instance, that is bold, but then ‘My Thesis’ is not bold, and I can’t work out why (and would like it to be). Can anyone tell me why only the blog parts bold when you’re on that page?

I’ve never touched WordPress, CSS or PHP before today, and while I’ve managed to tweak a few things, a solution to this is eluding me so far.

Thanks in advance!