This is a implementation of a [Base 16 Builder](https://github.com/chriskempson/base16/blob/main/builder.md) implemented in nix. First mentioned [here](https://github.com/nix-community/home-manager/issues/1005).

applyTemplate.nix

# author: wuyoli
# License: MIT

pkgs: template: theme:

with builtins;
with pkgs.lib;

let
  fromHex = hexStr:
    let
      upperHexStr = toUpper hexStr;
    in
      if stringLength upperHexStr == 0 then
        throw "fromHex can't handle empty strings"
      else if stringLength upperHexStr > 1000 then
        throw "fromHex can't handle strings longer than 1000 elements"
      else if stringLength upperHexStr == 1 then
        if match "[[:digit:]]" upperHexStr != null then
          strings.toInt upperHexStr
        else
          if upperHexStr == "A" then
            10
          else if upperHexStr == "B" then
            11
          else if upperHexStr == "C" then
            12
          else if upperHexStr == "D" then
            13
          else if upperHexStr == "E" then
            14
          else if upperHexStr == "F" then
            15
          else
            throw "string \"${hexStr}\" does not contain a hex value"
      else
        16 * (fromHex (substring 0 1 upperHexStr)) + (fromHex (substring 1 1001 upperHexStr));
  
  getColor = argList:
    # $1: hex code of 
    let
      colorNr = head argList;
      encoding = head (tail argList);
      parameter = strings.toLower (head (tail (tail argList)));
      hexRGB = theme.colors."base0${colorNr}";
      hex = {
        r = substring 0 2 hexRGB;
        g = substring 2 2 hexRGB;
        b = substring 4 2 hexRGB;
      };
    in
      if encoding == "hex-bgr" then
        hex.b ++ hex.g ++ hex.r
      else if encoding == "hex" then
        if parameter != null then
          "hex${parameter}"
        else
          templateColor
      else if encoding == "rgb" then
        toString (fromHex hex."${parameter}")
      else
        throw "Something went wrong while converting the colors to the proper format.";

  getValue = str:
    if str == "scheme-name" then
      theme.meta.name
    else if str == "scheme-author" then
      theme.meta.author
    else if str == "scheme-slug" then
      builtins.replaceStrings [ " " ] [ "-" ] (builtins.toLower theme.meta.author)
    else if substring 0 1 str == "!" then # comments
      ""
    else
      let
        captured = builtins.match "base0([0-F])-(hex-bgr|hex|rgb|dec|hsl)-?([rgbhsl]?)" str;
      in
        if captured != null then
          getColor captured
        else
          throw "theme template contains illegal variable: ${str}";

  splitList = split "\\{\\{([^}]+)}}" template;

  replacedList = map (elem: if isString elem then elem else getValue (head elem)) splitList;

in
  strings.concatStrings replacedList

theme.nix

{
  meta = {
    name = "OneDark";
    author = "Lalit Magant (http://github.com/tilal6991)";
  };

  colors = {
    base00 = "282c34";
    base01 = "353b45";
    base02 = "3e4451";
    base03 = "545862";
    base04 = "565c64";
    base05 = "abb2bf";
    base06 = "b6bdca";
    base07 = "c8ccd4";
    base08 = "e06c75";
    base09 = "d19a66";
    base0A = "e5c07b";
    base0B = "98c379";
    base0C = "56b6c2";
    base0D = "61afef";
    base0E = "c678dd";
    base0F = "be5046";
  };
}