Groovy Lambda is a simple VS Code color theme optimised for Haskell, based on Gruvbox Material Dark.

Supports the TextMate scopes provided by the Language Haskell VS Code extension:

• disambiguates between data constructors and types in data declarations,
• visually clear appearance of qualified names,
• distinguishes different uses of @: type application, record syntax or infix operator,
• special highlighting for record fields,
• highlights MagicHash and OverloadedLabels,
• underlines recognised GHC language extensions.

The theme can be installed directly from VS Code: search for Groovy Lambda in the extensions, and install.
See also the its VS Code marketplace page.

To change how something is highlighted, proceed as follows:

• While browsing code containing highlighting you'd like to change, start the TextMate scope inspector as follows:
  • open the VS Code Command Palette (Ctrl+Shift+P),
  • start typing "Inspect Editor Tokens and Scopes" and select this item from the dropdown menu.
• Click on the code whose highlighting you'd like to change; this will display the TextMate scopes associated to that piece of code. For instance, if in a Haskell source file is given the scope keyword.control.if.haskell.
• Open your settings.json file:
  • open the VS Code Command Palette (Ctrl+Shift+P),
  • start typing "Open Settings (JSON)" and select this item from the dropdown menu.
• Make the modifications by editing the value of editor.tokenColorCustomizations in your settings. For instance, to highlight if and then in red and else in blue, one would write:

    "editor.tokenColorCustomizations": {
      "textMateRules": [
        {
          "scope": [ "keyword.control.if", "keyword.control.then" ],
          "settings": {
            "foreground": "#ff0000"
          }
        },
        {
          "scope": "keyword.control.else",
          "settings": {
            "foreground": "#0000ff"
          }
        }
      ]
    }