Fix missing shell code highlighting

This now applies a default style to <pre> elements with <code> elements inside.

I did not use ```shell because it looks ugly.

static/css/post.css

@@ -97,6 +97,11 @@
     word-break: break-word;
 }
 
+.post-content>pre:has(code) {
+    color: #cdd6f4;
+    background-color: #1e1e2e;
+}
+
 .post-content img {
     margin: auto;
     max-height: 70vh;