Nojs: add `background-color` CSS to all iframes

So if they are popped out their background looks like it should (and not
pure white).

anonstream/templates/nojs_chat_form.html

@@ -21,6 +21,7 @@
       height: 100%;
       color: #ddd;
       font-family: sans-serif;
+      background-color: #232327;
     }
     a {
       color: var(--link-color);

anonstream/templates/nojs_chat_messages.html

@@ -24,6 +24,7 @@
       font-family: sans-serif;
       font-size: 11pt;
       transform: rotate(180deg);
+      background-color: #232327;
     }
 
     .button {

anonstream/templates/nojs_chat_users.html

@@ -18,6 +18,7 @@
       margin: 0;
       color: #ddd;
       font-family: sans-serif;
+      background-color: #121214;
     }
     #timeout {
       height: 0;

anonstream/templates/nojs_info.html

@@ -15,6 +15,7 @@
       margin: 0.75ch 1.25ch;
       font-family: sans-serif;
       color: #ddd;
+      background-color: #18181a;
     }
     #float {
       float: right;

anonstream/templates/nojs_stream.html

@@ -18,6 +18,7 @@
       overflow: hidden;
       color: #ddd;
       font-family: sans-serif;
+      background-color: black;
     }
     {% if online %}
       #video {