Improve logo image display in different themes

src/librustdoc/html/static/themes/ayu.css

@@ -61,6 +61,10 @@ pre {
 	background-color: #14191f;
 }
 
+.logo-container > img {
+	filter: drop-shadow(0 0 5px #fff);
+}
+
 /* Improve the scrollbar display on firefox */
 * {
 	scrollbar-color: #5c6773 transparent;

src/librustdoc/html/static/themes/dark.css

@@ -32,6 +32,10 @@ pre {
 	background-color: #505050;
 }
 
+.logo-container > img {
+	filter: drop-shadow(0 0 5px #fff);
+}
+
 /* Improve the scrollbar display on firefox */
 * {
 	scrollbar-color: rgb(64, 65, 67) #717171;

src/librustdoc/html/static/themes/light.css

@@ -43,6 +43,10 @@ pre {
 	scrollbar-color: rgba(36, 37, 39, 0.6) #d9d9d9;
 }
 
+.logo-container > img {
+	filter: drop-shadow(0 0 5px #aaa);
+}
+
 /* Improve the scrollbar display on webkit-based browsers */
 ::-webkit-scrollbar-track {
 	background-color: #ecebeb;