Change highlight colours.

[caz]

I'm currently using the Cinnamon desktop and I've been trying to change the colour of the highlighted item - in the menus list of various applications - and have managed to change them from green to blue which I'm happy with. But I've found that the search box in my home folder and the layers list in Gimp are still using a green colour which I would like to change to a darkish grey - this would be the same colours I get when using the Mate desktop.
I've had a good search around but can't seem to find a way to do this, if anyone knows a way to do this it will be much appreciated.
Also at the top of the Gimp window the fonts to list item such as File, Window, Edit and so on are quite small, if fact they look smaller than in any other application. I've tried changing the size of them by using Cinnamon settings and then the fonts tab but although I can change the font sizes for everything else the ones at the top of the Gimp window stay the same, so I'd be grateful for any ideas or thoughts on how to solve this problem.
Cheers.
Btw I'm using Mate 14

[caz]

By way of a bit of an update I've noticed that in some GTK themes the fonts display at a larger size than in others, this is despite the fact that they are listed as being the same size, usually a sans 10, when I check using xlappearance. I've had a look into some of the gtkrc files to see if there's a way to change font sizes in there but so far I've not been able to find any lines listing the font sizes. Also if I do try to change any of the values I am prevented from doing so - hitting the delete key makes no changes. I'm not sure if I need to log in as root to make changes to the gtkrc file as there's a red cross placed next to the file name in the top left corner of the gedit window... just noticed that the file is read only so I'm stuck on how to make any changes.
I've seen a few tutorials on how to make changes to the gtkrc files but they all seem pretty old and not that clear for someone with a limited understanding of how to go about making changes to a theme, so I'm wondering if anyone knows of any tutorials that might be similar in nature to the ones I found for making changes to the conky file. Also it would be great if there are any basic theme files that can be downloaded on and experimented with, I always find the best way to learn anything is by trail and error.
Thanks.