feat: add theme

shamelessly stolen from github:jnsgruk/nixos-config

Signed-off-by: Lander Van den Bulcke <landervandenbulcke@gmail.com>

lib/theme/lib.nix

@@ -0,0 +1,68 @@
+{ lib, ... }:
+with lib;
+rec {
+  # color-related functions
+
+  # convert rrggbb hex to #rrggbb
+  x = c: "#${c}";
+
+  # convert #rrggbb -> r, g, b
+  hexToRgb =
+    c:
+    let
+      r = toString (hexToDec (__substring 1 2 c));
+      g = toString (hexToDec (__substring 3 2 c));
+      b = toString (hexToDec (__substring 5 2 c));
+      res = "${r}, ${g}, ${b}";
+    in
+    res;
+
+  # functions copied from https://gist.github.com/corpix/f761c82c9d6fdbc1b3846b37e1020e11
+  # convert a hex value to an integer
+  hexToDec =
+    v:
+    let
+      hexToInt = {
+        "0" = 0;
+        "1" = 1;
+        "2" = 2;
+        "3" = 3;
+        "4" = 4;
+        "5" = 5;
+        "6" = 6;
+        "7" = 7;
+        "8" = 8;
+        "9" = 9;
+        "a" = 10;
+        "b" = 11;
+        "c" = 12;
+        "d" = 13;
+        "e" = 14;
+        "f" = 15;
+        "A" = 10;
+        "B" = 11;
+        "C" = 12;
+        "D" = 13;
+        "E" = 14;
+        "F" = 15;
+      };
+      chars = stringToCharacters v;
+      charsLen = length chars;
+    in
+    foldl (a: v: a + v) 0 (imap0 (k: v: hexToInt."${v}" * (pow 16 (charsLen - k - 1))) chars);
+
+  pow =
+    let
+      pow' =
+        base: exponent: value:
+        # FIXME: It will silently overflow on values > 2**62 :(
+        # The value will become negative or zero in this case
+        if exponent == 0 then
+          1
+        else if exponent <= 1 then
+          value
+        else
+          (pow' base (exponent - 1) (value * base));
+    in
+    base: exponent: pow' base exponent base;
+}