doc: Add NixOS and preview instructions

INSTALLATION.md

@@ -2,14 +2,29 @@
 
 > [!CAUTION]
 > Before installation on hardware it is recomended to practice in a VM and having a rescue USB stick ready with your
-> Linux distribution. Theme installation is a safe process if done right, but you better be safe than sorry.
+> Linux distribution. Theme installation is a safe process if done right, but better be safe than sorry.
 
 > [!TIP]
 > If you want to see how the theme looks before you install it, there are some methods to do so in
 > [showcase](SHOWCASE.md).
 <!-- TODO: Link to the showcase header -->
 
-## Generic instructions
+## NixOS (Flakes)
+
+1. Add this repo as a flake input.
+2. Set [`boot.loader.grub.theme`](https://search.nixos.org/options?channel=24.11&show=boot.loader.grub.theme&from=0&size=50&sort=relevance&type=packages&query=boot.loader.grub+theme) 
+
+   ```nix
+   { inputs, ... }: {
+      boot.loader.grub.theme = let
+        colorsheme = "night";
+        layout = "teleport";
+        resolution = "1920x1080";
+      in inputs.grubshin-bootpact.${colorsheme}.${layout}.${resolution};
+   }
+   ```
+
+## Other Linux distributions
 
 1. Download a [release from GitHub](https://github.com/max-ishere/grubshin-bootpact/releases/latest).
 2. If you have not installed themes before, you might have to [enable themes in GRUB](#enable-themes)

SHOWCASE.md

@@ -3,6 +3,31 @@
 > [!NOTE]
 > Scroll down for screenshots.
 
+## Previewing the themes
+
+### NixOS
+
+If you're on Nix you just have to run one command:
+
+```sh
+nix build "github:max-ishere/grubshin-bootpact#$colors.$layout.$resolution.preview-theme"
+
+# Example
+nix build "github:max-ishere/grubshin-bootpact#night.teleport.1920x1080.preview-theme"
+```
+
+Where colors, layout and resolution can be obtained from the tables below.
+
+This will create a result directory with an ISO file and a script to boot it in a QEMU VM. You can flash the ISO on a
+USB as well, but you cannot use it to boot your OS because the only entries it has are showcasing the icons with a
+reboot command.
+
+### Other linux distributions
+
+`hartwork` made an awesome tool <https://github.com/hartwork/grub2-theme-preview>. This lets you generate a preview ISO
+as well and run it in QEMU. In fact, the NixOS approach replicates the majority of the functionality of this tool, the
+only difference is that it is implemented in Nix and not python.
+
 ## Supported resolutions
 
 | `nix build` | Common name |
@@ -27,7 +52,6 @@
 
 ## Screenshots
 
-
 ### Teleport (Night)
 
 <img src="screenshots/teleport-night-720.png" />
@@ -50,4 +74,4 @@
 
 ### Classic (Abyss)
 
-<img src="screenshots/classic-abyss-720.png" />
+<img src="screenshots/classic-abyss-720.png" />