DarkOnyx.css

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/* CSS placed here will affect users of the Onyx night theme */

/* Header background */
#onyx-header-wikiHeader {
	background: linear-gradient(to left, transparent, #272727), url(https://halo.wiki.gallery/images/8/8c/Onyx-background-header.png) repeat-y;
	background-position: right;
	background-size: 500px;
}

/* Page background */
#onyx-page {
	background-color: #272727;
}
#onyx-page-pageBody {
	background: url(https://halo.wiki.gallery/images/c/cc/Onyx-watermark.png) no-repeat center top;
}
pre {
	background: rgba(20, 20, 20, 0.5) !important;
}

/* Font color */
body, h1, h2, h3, h4, h5, h6 {
	color: #fff;
}

/* Blue buttons */
input[type="submit"], input[type="button"], input[type="reset"], input[type="file"] {
	color: #fff;
	cursor: pointer;
	background-color: #224e96;
	background-image: -moz-linear-gradient(top,#2b64c0 35%,#224e96 65%);
	background-image: -webkit-gradient(linear,0% 0%,0% 100%,color-stop(35%,#2b64c0),color-stop(65%,#224e96));
	background-image: -o-linear-gradient(top,#2b64c0 35%,#224e96 65%);
	background-image: -ms-linear-gradient(top,#2b64c0 35%,#224e96 65%);
	border: 1px solid #224e96;
	border-radius: 4px;
}

/* Standard changes */

#onyx-navigation-list > li > a {
	color: #fff !important;
}

.mw-ui-input {
	border-color: #666 !important;
}
.mw-ui-input, .mw-ui-button {
	box-shadow: inset 0 0 0 0.1em #333 !important;
}
.mw-ui-button:hover {
	color: #a2a9b1 !important;
}

MediaWiki:DarkOnyx.css