docs: Improve theming + introduction

src/routes/getting-started/css.md

@@ -0,0 +1,33 @@
+<svelte:head>
+    <title>CSS - Kleos</title>
+</svelte:head>
+
+# CSS Configuration
+
+Use [PostCSS](https://postcss.org/) to process your css in components and `.postcss` files.
+
+Install PostCSS with:
+
+```bash
+pnpx svelte-add@latest postcss
+```
+
+This is the recommended `postcss.config.cjs` setup:
+
+```js
+const config = {
+  plugins: [
+    require('postcss-import'),
+    require('postcss-strip-inline-comments'),
+    require('postcss-nested'),
+    require('postcss-preset-env'),
+    require('autoprefixer'),
+    require('postcss-extend-rule'),
+    process.env.NODE_ENV === 'development' && require('cssnano')({
+      preset: 'default',
+    })
+  ],
+};
+
+module.exports = config;
+```