Index: src/wp-admin/css/common.css
===================================================================
--- src/wp-admin/css/common.css	(revision 33859)
+++ src/wp-admin/css/common.css	(working copy)
@@ -2603,12 +2603,22 @@
 }

 /* Thickbox for Plugin Install screen */
-body.plugin-install-php #TB_window,
-body.import-php #TB_window,
-body.plugins-php #TB_window,
-body.update-core-php #TB_window,
-body.index-php #TB_window {
-	background: #fcfcfc url( ../images/spinner.gif ) no-repeat center;
+body.plugin-install-php #TB_window:before,
+body.import-php #TB_window:before,
+body.plugins-php #TB_window:before,
+body.update-core-php #TB_window:before,
+body.index-php #TB_window:before {
+	content: '';
+	display: block;
+	width: 20px;
+	height: 20px;
+	position: absolute;
+	left: 50%;
+	top: 50%;
+	z-index: -1;
+	margin: -10px 0 0 -10px;
+	transform: translateZ(0);
+	background: #fcfcfc url( ../images/spinner.gif ) no-repeat center;
 }

 @media print,
@@ -2615,11 +2625,11 @@
 	(-webkit-min-device-pixel-ratio: 1.25),
 	(min-resolution: 120dpi) {

-	body.plugin-install-php #TB_window,
-	body.import-php #TB_window,
-	body.plugins-php #TB_window,
-	body.update-core-php #TB_window,
-	body.index-php #TB_window {
+	body.plugin-install-php #TB_window:before,
+	body.import-php #TB_window:before,
+	body.plugins-php #TB_window:before,
+	body.update-core-php #TB_window:before,
+	body.index-php #TB_window:before {
 		background: #fcfcfc url( ../images/spinner-2x.gif ) no-repeat center;
 		-webkit-background-size: 20px 20px;
 		background-size: 20px 20px;