Have just sorted out my system so it updates correctly, thank you ZeroZero, and decided to alter the themes. On clicking on the get more themes link within the Cinnamon Set window, the system starts opening up the Office suite Libre! I've checked the url and copied and pasted it, which is fine, but is this a glitch in the code or have I managed to somehow change the default program for opening html files. Most other links seem to work fine, so I'm not sure what's going on here.
Suggestions for things to consider/try welcome.