Is there any tool to give me the merged version of two .css files?

Question

I have two .css files that I want to merge them. Something like what git dies. All I'm looking for is a tool like git which gets two files as input and generates a file as output which is the merged version of those two input files.

Is there such a thing?

---

For Example:

/* file1.css */
.header{
    background-color: #f1f1f1;
    font-size: 13px;
}


/* file2.css */
.header{
    background-color: #f1f1f1;
}
.footer{
    background-color: gray;
    position: reletive;
}


/* expected_result.css */
.header{
    background-color: #f1f1f1;
    font-size: 13px;
}
.footer{
    background-color: gray;
    position: reletive;
}