Changing the Background color for image captions

style.css of your hteme, near the end:

.wp-caption {
	border: 1px solid #ddd;
	text-align: center;
	background-color: #f3f3f3;
	padding-top: 4px;
	margin: 10px;
	-moz-border-radius: 3px;
	-khtml-border-radius: 3px;
	-webkit-border-radius: 3px;
	border-radius: 3px;
}

https://www.w3schools.com/css/css_background.asp

for fundamental formatting problems, try to use a browser tool such as Firebug https://getfirebug.com/ to help identifying the involved css selectors.

Thank you alchymyth The Sweeper

You are a gentleman and a scholar