lets say in
a.css (first .css) i have
body{ background: yellow}
p { color: blue}
span { background : red}
h2{ color:orange}
b.css (second .css) i have
body{ background: red}
p { color: blue}
span { background : black}
h1{ font-weight:bold}
c.css (a+b.css) should have
body{ background: red}
p { color: blue}
span { background : black}
h1{ font-weight:bold}
h2{ color:orange}
Very simple. basically c.css gets a.css and b.css and overrides all the properties existing in a.css and b.css,
This task was easy but what if we have .css files with thousands of lines? is there any software that will aoutomat this task?
Have you looked at http://www.cssoptimiser.com/?
-- EDIT --
http://iceyboard.no-ip.org/projects/css_compressor generates:
h2{color:orange}body{background:red}p{color:blue}span{background:black}h1{font-weight:700}
which matches your expected output.