docs: Improve theming + introduction

src/routes/getting-started/css.md

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+<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;
+```

src/routes/getting-started/icons.md

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+<script>
+      import IconHeart from 'virtual:icons/heroicons-outline/heart'
+</script>
+
+<svelte:head>
+    <title>Icons - Kleos</title>
+</svelte:head>
+
+# Icons
+
+Use [unplugin-icons](https://github.com/antfu/unplugin-icons) to import icons as Svelte components.
+
+## Installation
+
+Install the package with:
+
+```bash
+pnpm add -D unplugin-icons
+```
+
+Import the plugin in your `svelte.config.js` file:
+
+```js
+import Icons from 'unplugin-icons/vite';
+
+const config = {
+    kit: {
+        vite: {
+            plugins: [
+                Icons({
+                    compiler: 'svelte',
+                }),
+            ],
+        },
+    },
+};
+```
+
+## Choosing icons
+
+`heroicons-outline` is the most used icon pack, and should be preferred above all others. If you can't find a good icon, or are looking for an alternative, use `lucide`, `fa-regular` (FontAwesome), `heroicons-solid`, and `carbon`, in that order. [Browse icons...](https://icones.js.org/collection/heroicons-outline)
+
+To install an icon pack, run:
+
+```bash
+pnpm add -D @iconify-json/heroicons-outline
+```
+
+Replacing `heroicons-outline`, with the pack you are trying to install.
+
+## Using icons
+
+Import an icon in the `<script>` tag of your component:
+
+```js
+  import IconHeart from 'virtual:icons/heroicons-outline/heart'
+```
+
+Then use the icon as if it were a Svelte component:
+
+> The `height` and `width` props are optional, and take CSS compatible values
+
+```html
+That's lovely <IconHeart height="14px" />!
+```
+
+That's lovely <IconHeart height="14px" />!