Redo Color Theme and css cleanup

* remove vars and add elements to base and btn vars
* change default border radius to 10px and padding to 0.7em
* put border radius and padding on search input form, infoxbox and buttons
* remove unused .help class in #categories_container
* remove active background from tabs to straemline design
* redo search form: 10px padding
* 2rem margin on search results on desktop
* fix modal pacement of engine reliability in prefs
* use darker accent colors
* streamline autocomplete with more padding and a hover effect

searx/static/themes/simple/src/less/preferences.less

@@ -1,9 +1,3 @@
-.column-reliability {
-  .engine-tooltip {
-    right: 12rem;
-  }
-}
-
 #tab-contentquery table td,
 #tab-contentquery table th {
   text-align: left !important;
@@ -83,15 +77,15 @@
   }
 
   .preferences_back {
-    background: none repeat scroll 0 0 var(--color-settings-return-background);
-    color: var(--color-settings-return-font);
+    background: none repeat scroll 0 0 var(--color-btn-background);
+    color: var(--color-btn-font);
     border: 0 none;
     .rounded-corners;
 
     cursor: pointer;
     display: inline-block;
     margin: 2px 4px;
-    padding: 0.5em;
+    padding: 0.7em;
 
     a {
       display: block;