Cannot Override Index to Resize Header

Hi,

I am trying to resize my header so that the image spans 100% of the page. Currently, the Nisarg theme makes the header much too large for display in a standard browser window, even though the image is the requested size of 1400 x 400 px. You can see the site here. Using the inspector tool in Chrome, I’ve figured out the fix is the following:

@media (max-width: 2000px) and (min-width: 10px)
.site-header {
background-size: 100% 100%;
height: 250px;
}

However, the style.css and child style.css are being overridden by what Chrome calls “(index)” not to be confused with index.php. I have no idea what (index) is or how to modify or override the .site-header code that seems to have precedent over my style.css code.

Little help?! How can I make the above code take effect?!