I have created a accordian menu in sidebar 1 and want to change the style, make the font bold and change the font colour. Is this something that I should do in the "Custom.css" file?

The menu in qestion can be seen here:
uk605.directrouter.com/~hrwsgjpz/index.php/effects

Yes you would need to change this via the custom.css

Are you looking to change the toggle link text or the links?

Cheers
Paul

I was hoping to get them same as the main menu which is black and blue with the font being bolder.

Try

.zen-nav.zen-nav-stacked li a:not(.btn), .zen-nav.zen-nav-stacked li span {color: #111;font-weight: bold;}

for the link

For the hover it needs to be a little different

#grid9 a:hover, #grid9 .zen-menu-heading:hover {color: #3d69d6;}

Cheers Paul,
Worked a treat!

A week or so ago I read in another thread you were saying to hold off upgrading to Joomla 3.7, does that advice still stand or would you suggest it is okay now?

There is a Joomla update pending for tomorrow

So I'd leave it till that is released and read the doc on it first

Joomla! Announcements

Joomla! 3.7.1 - Important Security Announcement - Patch Available Soon

A Joomla! 3.7.1 release containing a security fix will be published on Wednesday 17th May at approximately 14:00 UTC. The Joomla! Security Strike Team (JSST) has been informed of a critical security issue in the Joomla! core.

www.joomla.org/announcements/release-news/5704-important-security-announcement-pre-release-371.html

As Always, Many Thanks!

No worries

I do update my joomla sites if its a security update but I tend to wait on other updates a bit

Cheers
Paul