search_box: Fix search bar background on nightmode.

Commit c4e59309e4 introduced a
regression that caused a small part of the navbar in night mode to not
have the correct background color.

The relevant changes in that commit intended to fix the margin for the
search box for when the search pills feature was set to active.

This commit slightly increases the padding for the search box (when
pills are active), to improve pill alignment, and adds styles for
"#searchbox_legacy" to correct the background when search pills are
disabled.

This also reverts the change from commit
29b8e11e20 which tried to improve the
alignment of pills by adding a margin left but didn't address the
background color issue.

static/styles/zulip.scss

@@ -1654,7 +1654,6 @@ div.focused_table {
         font-family: 'Source Sans Pro';
         font-weight: 300;
         line-height: $header_height;
-        margin-left: 10px;
     }
 
     #search_arrows:focus {
@@ -1705,7 +1704,7 @@ div.focused_table {
     }
 
     #search_arrows {
-        padding-left: 30px;
+        padding-left: 35px;
         font-size: 90%;
         letter-spacing: normal;
     }
@@ -1732,6 +1731,15 @@ div.focused_table {
     }
 }
 
+#searchbox_legacy {
+    #search_arrows {
+        padding-left: 0px;
+    }
+    #search_query {
+        padding-left: 35px;
+    }
+}
+
 #tab_bar_underpadding {
     position: absolute;
     width: 100%;