diff --git a/README.md b/README.md
index 1f779002..5fe3d382 100644
--- a/README.md
+++ b/README.md
@@ -55,9 +55,12 @@ a NixOS module; how to do this is shown in the example above.
 { pkgs, ... }:
 
 {
-  # A colorscheme will be chosen automatically based on your wallpaper
+  # A colorscheme will be chosen automatically based on your wallpaper.
   stylix.image = ./wallpaper.png;
 
+  # Use this option to force a light or dark theme.
+  stylix.polarity = "light";
+
   # Select your preferred fonts, or use these defaults:
   stylix.fonts = {
     serif = {
diff --git a/palette-generator/Stylix/Main.hs b/palette-generator/Stylix/Main.hs
index 1448b6dc..7b5ffee7 100644
--- a/palette-generator/Stylix/Main.hs
+++ b/palette-generator/Stylix/Main.hs
@@ -11,10 +11,11 @@ import Text.JSON ( encode )
 
 -- | Run the genetic algorithm to generate a palette from the given image.
 selectColours :: (Floating a, Real a)
-              => V.Vector (LAB a) -- ^ Colours of the source image
+              => String -- ^ Scheme type: "either", "light" or "dark"
+              -> V.Vector (LAB a) -- ^ Colours of the source image
               -> V.Vector (LAB a) -- ^ Generated palette
-selectColours image
-  = snd $ evolve image (EvolutionConfig 1000 100 0.5 150) (mkStdGen 0)
+selectColours polarity image
+  = snd $ evolve (polarity, image) (EvolutionConfig 1000 100 0.5 150) (mkStdGen 0)
 
 -- | Convert a 'DynamicImage' to a simple 'V.Vector' of colours.
 unpackImage :: (Num a) => DynamicImage -> V.Vector (RGB a)
@@ -30,22 +31,23 @@ loadImage :: String -- ^ Path to the file
           -> IO DynamicImage
 loadImage input = either error id <$> readImage input
 
-mainProcess :: (String, String) -> IO ()
-mainProcess (input, output) = do
+mainProcess :: (String, String, String) -> IO ()
+mainProcess (polarity, input, output) = do
   putStrLn $ "Processing " ++ input
   image <- loadImage input
   let outputTable = makeOutputTable
                   $ V.map lab2rgb
-                  $ selectColours
+                  $ selectColours polarity
                   $ V.map rgb2lab
                   $ unpackImage image
   writeFile output $ encode outputTable
   putStrLn $ "Saved to " ++ output
 
-parseArguments :: [String] -> Either String (String, String)
-parseArguments [input, output] = Right (input, output)
-parseArguments [_] = Left "Please specify an output file"
-parseArguments [] = Left "Please specify an image"
+parseArguments :: [String] -> Either String (String, String, String)
+parseArguments [polarity, input, output] = Right (polarity, input, output)
+parseArguments [_, _] = Left "Please specify an output file"
+parseArguments [_] = Left "Please specify an image"
+parseArguments [] = Left "Please specify a polarity: either, light or dark"
 parseArguments _ = Left "Too many arguments"
 
 main :: IO ()
diff --git a/palette-generator/Stylix/Palette.hs b/palette-generator/Stylix/Palette.hs
index 64da6a1e..0b1d5974 100644
--- a/palette-generator/Stylix/Palette.hs
+++ b/palette-generator/Stylix/Palette.hs
@@ -34,12 +34,12 @@ randomFromVector generator vector
   = let (index, generator') = randomR (0, V.length vector - 1) generator
      in (vector ! index, generator')
 
-instance (Floating a, Real a) => Species (V.Vector (LAB a)) (V.Vector (LAB a)) where
+instance (Floating a, Real a) => Species (String, (V.Vector (LAB a))) (V.Vector (LAB a)) where
   {- |
   Palettes in the initial population are created by randomly
   sampling 16 colours from the source image.
   -}
-  generate image = generateColour 16
+  generate (_, image) = generateColour 16
       where generateColour 0 generator = (generator, V.empty)
             generateColour n generator
               = let (colour, generator') = randomFromVector generator image
@@ -51,17 +51,13 @@ instance (Floating a, Real a) => Species (V.Vector (LAB a)) (V.Vector (LAB a)) w
   Mutation is done by replacing a random slot in the palette with
   a new colour, which is randomly sampled from the source image.
   -}
-  mutate image generator palette
+  mutate (_, image) generator palette
     = let (index, generator') = randomR (0, 15) generator
           (colour, generator'') = randomFromVector generator' image
        in (generator'', palette // [(index, colour)])
 
-  fitness _ palette
-    = realToFrac $
-      accentDifference -
-      -- Either light schemes or dark themes are allowed.
-      -- We try to converge on whichever theme we are closer to.
-      min lightScheme darkScheme
+  fitness (polarity, _) palette
+    = realToFrac $ accentDifference - scheme
       where
         -- The accent colours should be as different as possible.
         accentDifference = minimum $ do
@@ -79,6 +75,12 @@ instance (Floating a, Real a) => Species (V.Vector (LAB a)) (V.Vector (LAB a)) w
           -- The accent colours should all have the given lightness.
           + sum (V.map (difference accentValue) $ accent lightnesses)
 
+        scheme = case polarity of
+          "either" -> min lightScheme darkScheme
+          "light" -> lightScheme
+          "dark" -> darkScheme
+          _ -> error ("Invalid polarity: " ++ polarity)
+
         {-
         For light themes, the background is bright and the text is dark.
         The accent colours are slightly darker.
diff --git a/stylix/palette.nix b/stylix/palette.nix
index 46da7e51..fabdd5f7 100644
--- a/stylix/palette.nix
+++ b/stylix/palette.nix
@@ -7,7 +7,7 @@ let
   cfg = config.stylix;
 
   paletteJSON = pkgs.runCommand "palette.json" { } ''
-    ${palette-generator}/bin/palette-generator ${cfg.image} $out
+    ${palette-generator}/bin/palette-generator ${cfg.polarity} ${cfg.image} $out
   '';
 
   palette = importJSON paletteJSON // {
@@ -18,6 +18,18 @@ let
 
 in {
   options.stylix = {
+    polarity = mkOption {
+      type = types.enum [ "either" "light" "dark" ];
+      default = "either";
+      description = ''
+        Use this option to force a light or dark theme.
+
+        By default we will select whichever is ranked better by the genetic
+        algorithm. This aims to get good contrast between the foreground and
+        background, as well as some variety in the highlight colours.
+      '';
+    };
+
     image = mkOption {
       type = types.coercedTo types.package toString types.path;
       description = ''