Rename content slot to main, remove h1 from header for better

accessibility, and unify SVG fill color to #fff

Update BaseLayout to use a header <p> for the website title, rename the
main slot/id to
"main", and update all pages to match. Add CSS rule svg { fill:
currentColor }
and size header > p the same as h1. Fix a minor comment typo.

src/pages/parallax.astro

@@ -130,7 +130,7 @@ import Layout from "@layouts/BaseLayout.astro";
       }
     }
   </style>
-  <Fragment slot="content">
+  <Fragment slot="main">
     <article id="hero" x-data="{ theme: 'winternight'}">
       <div class="parallax-container" :class="'parallax-theme-' + theme">
         <div class="layer layer-0"></div>