Add styles for .tile-menu

This is currently used for the "submit" page, but hey, maybe we'll
end up using it elsewhere too since it's now so convenient to
implement.

assets/css/screen.css

@@ -95,6 +95,48 @@ body {
     width: 100%;
 }
 
+/* 3. Special Templates
+/* ---------------------------------------------------------- */
+
+/* 3.1 Tile Menu
+/* ---------------------------------------------------------- */
+
+.tile-menu {
+    display: grid;
+    grid-template-columns: repeat(auto-fit, minmax(16em, 1fr));
+    grid-gap: 1em;
+    grid-auto-rows: minmax(20rem, auto);
+    color: var(--color-darkgrey);
+}
+
+.tile-menu .tile {
+    display:block;
+    border: 1px solid var(--color-lightgrey);
+    padding: 2rem;
+    color: inherit;
+    text-decoration: none;
+}
+
+.tile-menu .tile p {
+    font-size: 1em;
+    line-height: 1.2em;
+    font-family: var(--font-sans);
+}
+
+.tile-menu .tile h2 {
+    font-size: 1.2em;
+    font-weight: bold;
+    text-align: center;
+    font-family: var(--font-heading);
+}
+
+html.dark-mode .tile-menu {
+    color: var(--color-lightgrey);
+}
+
+html.dark-mode .tile-menu .tile {
+    border-color: var(--color-darkgrey);
+}
 
 /* 4. Site Header
 /* ---------------------------------------------------------- */