Horizontal menu

…where would I put this? Inside the short code?

No, CMW does not provide any facility for specifying additional CSS rules. As to where you should put it … well, there are a number of possibilities:

• your theme may provide the facility for you to enter your own CSS rules as part of its custom settings; some themes do, most don’t
• if you are running a child theme, then you could edit that child’s stylesheet and enter the rules there
• there are plugins such as the CSS & Javascript Toolbox (and undoubtedly others similar) that provide the ability to enter your own CSS rules, above and beyond those of your theme
• lastly, you could modify your theme’s stylesheet, but I do not recommend this because any update to the theme would wipe out your customisations!

I assume 24 is the <id-of-top> referred to…

No, it isn’t. The 24 is the internal-to-WordPress id of the custom menu that the widget is configured to filter; this is not the same as the id attribute that WordPress assigns to the UL (or OL in your case) HTML element that gets output to the page. WordPress assigns a unique id of menu-{menu slug}[-#], where {menu slug} is the “name” of the custom menu being used, and [-#] is an optional, incremental number that ensures uniqueness when there are multiple occurrences of similar items being output on the one page. So, if your custom menu is called, say, “my custom menu”, then the first occurrence might have an id of menu-my-custom-menu, the 2nd would be menu-my-custom-menu-1, and the 3rd would be menu-my-custom-menu-2, etc.
There is a potential problem with using the WordPress-generated id though : if you subsequently insert another instance in front your original instance – the one you provide all the custom styling for – then the original instance (now the 2nd instance) will change its id from menu-my-custom-menu to menu-my-custom-menu-1 … which will throw out your styling. You might be better off providing your own unique class for either the widget itself, the container, or the list element (UL or OL), via the CMW settings. (Using a class rather than an id makes your CSS styles re-usable for other instances).

All the main browsers nowadays provide Developer facilities which enable you to play around with a page’s HTML & styling (to some degree, and with varying ease), and I would suggest that that is the place to start (personally, I could not do without Firefox’s Firebug extension). You can try out different CSS rules without having to code anything into WordPress, and only when you are happy with the results do you then have to make a decision as to how to implement them.

No problem, and I’m glad your theme is making things easier for you.