Minor tweaks

* use the background-color property instead of the background shorthand
* add multiple classes with one `addClass` call
* rename `underline` to `text-underline`
* use the CSS class `pointer` instead of adding CSS via JS
* change `border-color: transparent` to `border: 0` in the navbar right dropdown image
* add image dimensions in the dropdown donate image
* remove the obsolete and unneeded `center` element from network.php
* remove commented out PHP code from header.php
* fix wrong `form-control` class used in buttons

Signed-off-by: XhmikosR <xhmikosr@gmail.com>

style/pi-hole.css

@@ -261,6 +261,10 @@
 }
 
 /* Utilities */
+.text-underline {
+    text-decoration: underline;
+}
+
 .text-black {
     color: #000 !important;
 }
@@ -286,11 +290,10 @@
 }
 
 td.highlight {
-    background-color: yellow;
+    background-color: #ff0 !important;
 }
 
-code.breakall
-{
+code.breakall {
     white-space: -moz-pre-wrap;
     white-space: -pre-wrap;
     white-space: -o-pre-wrap;
@@ -299,10 +302,6 @@ code.breakall
     word-wrap: break-word; /* Internet Explorer 5.5+ */
 }
 
-.underline {
-    text-decoration: underline;
-}
-
 .pointer {
     cursor: pointer;
 }
@@ -310,4 +309,4 @@ code.breakall
 .bootstrap-select.bs-container.align-right {
     left: unset !important;
     right: 10px;
-}
+}