SharePoint Online menu drop-down widths

[randle]

Hi,

 

I have a school SharePoint Online site which is so far working well, however, the drop-down menus in the Oslo layout are too narrow and as a result wrap text if longer than one word. This has the undesirable outcome of looking a complete mess e.g. Computer Science is displayed as:

 

Computer

Science

 

With no dividers between the links this just looks shoddy!

 

I'm aware that I need to edit/create a CSS file but am struggling to see exactly which one and what to modify! I'm guessing the osol.html master page but some help would be appreciated.

 

Any ideas?