CSS problem

I would like to change the h1 background color. So I first try to input css in Mantra settings -> Miscellaneous settings -> Custom CSS box as follow:
.entry-content h1 {background-color:#ccc;}

but it does not work, the h1 background color is still white (default by mantra theme). I found, many settings made in “Custom CSS” box often cannot affect the page display based on my experiences for this theme. I don’t know why.

there are several files in mantra theme controlling the page display, such as style.css, custom-style.php, …, what kind of priority for these files.

I also try to create a child of mantra, and create a style.css file in this folder, put “@import url(“../mantra/style.css”);” at the top of the file to include the mantra style, and input all my custom css below. but the results are the same, no any changes.

any suggestion? Thank you very much.