chore: Couple design tweaks

src/pages/index.astro

···
 <Page title="Index">
   <header>
     <h1>
-      <svg viewBox="0 0 902 134" style="fill: currentColor; opacity: 0.375;" width="160">
+      <svg viewBox="0 0 902 134" width="160">
         <title>Diffuse</title>
         <use
           xlink:href="/images/diffuse-current.svg#diffuse"

src/styles/pages/index.css

···
 
 a {
   color: inherit;
+  text-underline-offset: 6px;
+}
+
+h1 svg {
+  fill: oklch(from var(--palm-leaf) calc(l + 0.125) c h);
 }
 
 h2 {
+  color: var(--beyond-the-pines);
   font-size: var(--fs-xl);
   font-weight: 900;
   margin: var(--space-xl) 0 var(--space-sm);