Fix CSS styling for warning admonitions

Now that warnings are wrapped in blockquotes, the CSS needs to be
adjusted for that.

This also includes a minor fix for the `::before` rule so that it only
applies to the first paragraph in a multi-paragraph warning.

theme/reference.css

@@ -17,36 +17,40 @@ Warnings are defined using admonitions in blockquotes:
 > This is bad!
 
 */
-main .warning p {
-    padding: 10px 20px;
-    margin: 20px 0;
+main .warning blockquote {
+    padding: 0px;
 }
 
-main .warning p::before {
+main .warning blockquote p {
+    padding: 0px 20px;
+    margin: 10px 0;
+}
+
+main .warning blockquote p:first-child::before {
     content: "⚠️ ";
 }
 
-.light main .warning p,
-.rust main .warning p {
+.light main .warning blockquote,
+.rust main .warning blockquote {
     border: 2px solid red;
     background: #ffcece;
 }
 
-.rust main .warning p {
+.rust main .warning blockquote {
     /* overrides previous declaration */
     border-color: #961717;
 }
 
-.coal main .warning p,
-.navy main .warning p,
-.ayu main .warning p {
+.coal main .warning blockquote,
+.navy main .warning blockquote,
+.ayu main .warning blockquote {
     background: #542626;
 }
 
 /* Make the links higher contrast on dark themes */
-.coal main .warning p a,
-.navy main .warning p a,
-.ayu main .warning p a {
+.coal main .warning blockquote p a,
+.navy main .warning blockquote p a,
+.ayu main .warning blockquote p a {
     color: #80d0d0;
 }