Add basic CSS for warning divs

Author: Havvy

src/theme/reference.css

@@ -77,3 +77,35 @@ the parenthetical. So for this example, you'd use
 .parenthetical {
     white-space: nowrap;
 }
+
+/*
+Warnings and notes:
+
+Write the <div>s on their own line. E.g.
+
+<div class="warning">
+
+Warning: This is bad!
+
+</div>
+*/
+main .warning p {
+    padding: 10px 20px;
+    margin: 20px 0;
+}
+
+.light main .warning p,
+.rust main .warning p {
+    border: 2px solid red;
+    background: #f2dede;
+}
+
+.coal main .warning p,
+.navy main .warning p,
+.ayu main .warning p {
+    background: #542626
+}
+
+main .warning p::before {
+    content: "⚠️ ";
+}