docs: add branding reference documentation for Kairos customization (#480)

Co-authored-by: Itxaka <1447686+Itxaka@users.noreply.github.com>
Co-authored-by: mudler <2420543+mudler@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: 3 people

.gitignore

@@ -1,5 +1,7 @@
 .hugo_build.lock
 node_modules/
 public/
+public_test/
+bin/
 .vscode/
 resources/

content/en/docs/Installation/interactive.md

@@ -19,3 +19,7 @@ When loading any Kairos ISOs, a GRUB menu, like the following will be displayed.
 ## Manually
 
 The interactive installer can be also started manually with `kairos-agent interactive-install` from the LiveCD.
+
+## Customization
+
+The interactive installer appearance and behavior can be customized through branding options. For details, see the [Branding](/docs/reference/branding) reference page.

content/en/docs/Reference/branding.md

@@ -0,0 +1,68 @@
+---
+title: "Branding"
+linkTitle: "Branding"
+weight: 12
+date: 2025-10-21
+description: Customize the appearance and behavior of Kairos components
+---
+
+Kairos supports branding customization to tailor the appearance and behavior of various components to match your organization's identity or requirements. Branding options are controlled through configuration files placed in the `/etc/kairos/branding/` directory.
+
+## Interactive Installer Branding
+
+The interactive installer supports customization options that allow you to modify its appearance and available options.
+
+### Color Scheme Customization
+
+You can customize the color scheme of the interactive installer by creating a file at `/etc/kairos/branding/interactive_install_colors`. This file should contain environment variable definitions for the colors you want to override. You can specify any, all, or none of these variables - they will override the corresponding default colors.
+
+The available color variables are:
+
+- `KAIROS_BG` - Background color
+- `KAIROS_TEXT` - Text color  
+- `KAIROS_HIGHLIGHT` - Primary highlight color
+- `KAIROS_HIGHLIGHT2` - Secondary highlight color
+- `KAIROS_ACCENT` - Accent color
+- `KAIROS_BORDER` - Border color
+- `CHECK_MARK` - Check mark character/symbol
+
+#### Color Format
+
+Colors can be specified in two formats depending on your terminal capabilities:
+
+**For full color terminals (24-bit/true color)**: Use hex triplet format in RGB:
+```bash
+# /etc/kairos/branding/interactive_install_colors
+KAIROS_BG="#03153a"        # Deep blue background
+KAIROS_TEXT="#ffffff"      # White text
+KAIROS_HIGHLIGHT="#e56a44" # Orange highlight
+KAIROS_ACCENT="#ee5007"    # Accent orange
+CHECK_MARK="✓"
+```
+
+**For simple/dumb terminals (16 colors)**: Use numbers 0-9 for basic colors:
+```bash
+# /etc/kairos/branding/interactive_install_colors
+KAIROS_BG="0"        # Black background
+KAIROS_TEXT="7"      # White text
+KAIROS_HIGHLIGHT="9" # Bright red highlight
+KAIROS_BORDER="9"    # Bright red border
+CHECK_MARK="*"
+```
+
+{{% alert title="Note" color="info" %}}
+If you set values to 0-9, those simple colors will be used even on 256-color terminals. For the best experience on modern terminals, use hex triplet format.
+{{% /alert %}}
+
+### Disabling Advanced Options
+
+If you want to hide the "Customize Further" option in the interactive installer, you can create an empty file at `/etc/kairos/branding/interactive_install_advanced_disabled`. When this file exists, the installer will only show the "Start Install" option, simplifying the interface for users who don't need advanced customization.
+
+```bash
+# Create the file to disable advanced options
+touch /etc/kairos/branding/interactive_install_advanced_disabled
+```
+
+### Applying Branding
+
+These branding files should be included in your Kairos image build process or deployed to the system before running the interactive installer. For details on building custom images, see the [Build from Scratch](/docs/reference/build-from-scratch) documentation.