Add background color for spoiler input, like toot textarea (#4181)

app/javascript/styles/components.scss

@@ -1748,6 +1748,7 @@
   width: 100%;
   margin: 0;
   color: $ui-base-color;
+  background: $simple-background-color;
   padding: 10px;
   font-family: inherit;
   font-size: 14px;
@@ -1770,7 +1771,6 @@
 
 .autosuggest-textarea__textarea {
   min-height: 100px;
-  background: $simple-background-color;
   border-radius: 4px 4px 0 0;
   padding-bottom: 0;
   padding-right: 10px + 22px;