You can customize the look and feel of all GUIs through this configuration. Changing the items used for the navigation bar to the colors used for items.

GUI Config Settings/GUIs.yml

Change Default Colors

# Global Color Settings
colors:
  # Title at the top of GUI
  title: "&{#42f5d4,#fff30a}"
  # Name color / prefix of the Item name inside GUI
  highlight: "&#fff30a▌ "
  # Secondary color / prefix in the Lore of the Item inside GUI
  lowlight: "&#42f5d4"

GUI Filler

# Fills in the empty spots of the GUI with this item
filler:
  type: "GRAY_STAINED_GLASS_PANE"
  name: "&0"

Navigation Elements

For custom GUIs you can change the position of fixed navigation items.

# Global Navigation Settings
navigation:
  # Fills in the blank spaces of the menu with this item
  filler:
    type: "BLACK_STAINED_GLASS_PANE"
    name: "&0"
  # Added to the left and right of the home button
  highlight:
    type: "CYAN_STAINED_GLASS_PANE"
    name: "&0"
    slot: ""
  # Used to go back in the nav to the previous GUI
  home:
    type: "NETHER_STAR"
    name: "Back"
    slot: ""
  # Item for "Next Page" icon in GUI
  nextPage:
    type: "SPRUCE_SIGN"
    name: "Next Page"
    slot: ""
  # Item for "Previous Page" icon in GUI
  previousPage:
    type: "OAK_SIGN"
    name: "Previous Page"
    model: ""
    slot: ""

Default Lore

# Customize the global lore
lore:
  # Shown on perk items that have an action
  footer:
    - ""
    - "[Tap to Use]"