I turned Less Compiler OFF, cleaned cache twice even in browser, but I still see /templates/gantry/css-compiled/master-17e205a6e5faa8557f5512b764b014ac.css links to css in Firebug.
How can I turn compression OFF?
You can't turn the compiler "OFF".
master-17e205a6e5faa8557f5512b764b014ac.css is the result of the compiling process.
You can turn off the compression of that output.
Sorry but I never told here I know what Less is.
And I just wondered why I can't see any effect by switching buttons of "Less Compiler" in adminpanel Advanced tab and what these buttons are for?
So finally as far as I understood the way to find right css-rule is to use firebug just for rule name finding and then search for each separate rule by hands in all of css files by NP++
Open your site.
Inspect the element with Firebug (or any other webinspector).
Firebug will tell you EXACTLY wich rules applied.
Just copy that rule into your custom CSS-file and chnage it for your needs (with proper CSS specificity ...)
Doing it this way you can update your site without losing changes.
Changing template files has always been a bad practice.