Hi, giupeppo - and welcome to the Linux Mint forums
I have also tried to do this - only change Mint-X green "highlights" to blue.
As you say, you can change some elements (in mate) by customising the Mint-X theme in Appearance Preferences, and try to change some elements
in gtkrc files. Yet some things still remain green.
I found that I had to actually change the color of the actual images in the theme folders using gimp. So I don't think it's a quick/easy fix.
For example, to change the "selection around "Documents"" as you want to change - you have to change the picture "row-selected.png" in the
Mint-X theme folder > gtk-2.0 > Apps > Caja to blue. This is just one of many
images throughout the folders I ended up changing like buttons and
checkboxes, as well as editing the gtkrc file and others.
I copied the original Mint-X folder from /usr/share/themes to /home/username/.themes, renamed the folder to Mint-blue and edited it there.
This ensures that the original Mint-X theme remains untouched and you can easily switch back if you don't like the results
I only set out to change those highlights but in the end, I suppose I have created a "new" theme with this result:
If you'd like to try it, you can download it. Maybe you can take it and tweak it further :http://www.mediafire.com/file/w46axa4kufn2q6j/Mint-blue.tar.gz
Edit: Just looked a couple of posts down in the forum unheeding
has also created a Mint-X based blue theme - you could try this as well.http://forums.linuxmint.com/viewtopic.php?f=211&t=126192