CSS: clean up some classes

There were some values hard-coded in the html attributes,
but we already were setting them in our css.

Also, search__backdrop was basically the same as search__outer__wrapper

sphinx_search/static/css/rtd_sphinx_search.css

@@ -2,26 +2,14 @@
     position: fixed;
     top: 0;
     left: 0;
-    width: 100%;
-    height: 100%;
-    z-index: 700;
-}
-
-/* Backdrop */
-
-.search__backdrop {
-    /* Positioning */
-    position: fixed;
-    top: 0;
-    left: 0;
-    z-index: 500;
 
     /* Display and box model */
     width: 100%;
     height: 100%;
     display: none;
 
     /* Other */
+    z-index: 700;
     background-color: rgba(0, 0, 0, 0.502);
 }
 

sphinx_search/static/js/rtd_sphinx_search.js

@@ -546,15 +546,15 @@ const fetchAndGenerateResults = (search_url, projectName) => {
  */
 const generateAndReturnInitialHtml = () => {
     let initialHtml =
-        '<div class="search__outer__wrapper search__backdrop"> \
+        '<div class="search__outer__wrapper"> \
             <div class="search__outer"> \
                 <div class="search__cross" title="Close"> \
                     <!--?xml version="1.0" encoding="UTF-8"?--> \
-                    <svg class="search__cross__img" width="15px" height="15px" enable-background="new 0 0 612 612" version="1.1" viewBox="0 0 612 612" xml:space="preserve" xmlns="http://www.w3.org/2000/svg"> \
+                    <svg class="search__cross__img" enable-background="new 0 0 612 612" version="1.1" viewBox="0 0 612 612" xml:space="preserve" xmlns="http://www.w3.org/2000/svg"> \
                         <polygon points="612 36.004 576.52 0.603 306 270.61 35.478 0.603 0 36.004 270.52 306.01 0 576 35.478 611.4 306 341.41 576.52 611.4 612 576 341.46 306.01"></polygon> \
                     </svg> \
                 </div> \
-                <input class="search__outer__input" placeholder="Search ..."> \
+                <input class="search__outer__input" placeholder="Search..."> \
                 <span class="bar"></span> \
             </div> \
             <div class="rtd__search__credits"> \