Bug 1588563 - Reduce background image size in about:config r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D49463

--HG--
extra : moz-landing-system : lando

browser/components/aboutconfig/content/aboutconfig.css

@@ -17,11 +17,11 @@
 }
 
 .config-background {
-  background-image: url("chrome://browser/content/aboutconfig/background.svg");
-  background-repeat: no-repeat;
-  background-position: center center;
-  background-size: 400px;
-  height: 350px;
+  background: url("chrome://browser/content/aboutconfig/background.svg") no-repeat;
+  height: 182px;
+  margin-block: 32px;
+  margin-inline: auto;
+  width: 235px;
 }
 
 .config-help-text {