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/toolkit.less

@@ -123,8 +123,8 @@ div.selectable_url {
   padding: 1em 1em 1em 2.7em;
   margin: 0 0 1em 0;
   border: 1px solid var(--color-toolkit-dialog-border);
-  border-radius: 4px;
   text-align: left;
+  .rounded-corners;
 
   &::before {
     position: absolute;