swerve

yes that is what I mean

Although it only works on the main category page – it does not work on the items inside the category
So I used

#mainNav .current-post-parent a { color: #fff; }

which changed the colour inside the category. I am unsure why though – if you could give me a brief explanation softsound solution (I am aware I am probably pushing my luck !) that would be greatly appreciated.

ah

#mainNav .current-menu-item a { color: #fff; }

was the one – thank you so much micheal and Softound Solutions
your advice is very much appreciated!

no that is not working – I have tried the following classes

.current-menu-item a:link { color: #fff; }

.menu-item-object-category a:link { color: #fff; }

.menu-item-type-taxonomy a:link { color: #fff; }

.menu-item-object-category a:link { color: #fff; }

and still nothing.
Any ideas?

Thank you micheal but I tried targeting that selector and it did not seem to work?

current-menu-item a:active {
color: #fff;
}

Does this look wrong to you?
Thank you again for your advice

Thanks you for getting back to me,
anyway , no I think there is a misunderstanding maybe I did not explain clearly – it is the whole header area not the just the logo space (which BTW centred layout option is really cool for mobile! ). I have a boxed layout – I think 1050 x 225 px is the size of the header area. I just need to know where to put the link for the image as I am not sure in the css (header.top ?) or the header file or even in the javascript?
This might make the sticky header look weird I know.
There is a file called custom header which has an option for a custom header but it is set in a php array which is very far outside of my limited knoowledge of php. Should I replace the header file with the custom header file in a child theme?
Anyway thank you again for getting back to me. I will definitely make a donation for writing and providing support for this theme