Can't custmise theme

I’m new to WP and coding but on a steep learning curve. I’m using the Expound theme. I was wanting to change the gallery text and image borders but can’t find the css code.

The stylesheet is this:

*/

@import url('css/reset.css');
@import url('css/expound.css?v=20140129');

There’s a rtl.css sheet as well with coding but it doesn’t seem to relate to general appearance. I did a CSS interrogation to help identify the relevant code which came up with:

<style type='text/css'>
                    #gallery-2 {
                    margin: auto;
                    }
                    #gallery-2 .gallery-item {
                    float: left;
                    margin-top: 10px;
                    text-align: center;
                    width: 33%;
                    }
                    #gallery-2 img {
                    border: 2px solid #cfcfcf;
                    }
                    #gallery-2 .gallery-caption {
                    margin-left: 0;
                    }
                    </style>
                    <!-- see gallery_shortcode() in wp-includes/media.php -->

After identifying “border: 2px solid #cfcfcf;” I then ran a “find” though the rtl.css code (in the absence of anywhere else to look) and could find anything matching. After reading up on templat hierarchy I decided in the absence of relevant code it was defaulting to WP.

Having already set up a Child theme in anticipation of being able to make some changes I input the following code string into the child theme style.css sheet:

img {
	border: 2px solid #000000;
}

When that didn’t work I figured maybe I’d set up the child theme incorrectly so I turned instead the Jetpack Custom CSS editor and tried the same code string there. Still nothing.

FYI…I can’t provide a link to the site as it is still in development and not live. I’m self-hosting not with WP.
Please help – this is driving me insane!