Using CSS with SharePoint 2013

I’m trying to change the background color to my web part title bars, but for some reason I can’t get my CSS modification to work.

First, I’ve made a copy of the default seattle.master and corev15.css files. I uploaded my new master file into the Master Page Gallery and pointed the site to it via the Site Master Page settings. I know that part is working, because as  test, I changed the favicon path and I do see that change.

Then, under the Alternate CSS URL, I’m pointing to my custom CSS file that I’ve uploaded to the Style Library. In addition to all of the CSS code included in the corev15.css file, I’ve added this code:

.ms-WPHeader TD{
 background-color:#0072c6
}

That doesn’t seem to do anything. After lots of Internet searching, I’ve also tried the alternative methods of pointing to the CSS file directly from the .master file AND putting that code directly on the page within an HTML editor web part. That doesn’t work either..

Any thoughts? This is driving me crazy…