diff --git a/Cabal/src/Distribution/Simple/Haddock.hs b/Cabal/src/Distribution/Simple/Haddock.hs
index f264f2342ea..a8bdc9acc67 100644
--- a/Cabal/src/Distribution/Simple/Haddock.hs
+++ b/Cabal/src/Distribution/Simple/Haddock.hs
@@ -618,6 +618,7 @@ fromHaddockProjectFlags flags =
     , argInterfaces = fromFlagOrDefault [] (haddockProjectInterfaces flags)
     , argLinkedSource = Flag True
     , argResourcesDir = haddockProjectResourcesDir flags
+    , argCssFile = haddockProjectCss flags
     }
 
 fromPackageDescription :: HaddockTarget -> PackageDescription -> HaddockArgs
diff --git a/cabal-install/src/Distribution/Client/CmdHaddock.hs b/cabal-install/src/Distribution/Client/CmdHaddock.hs
index 8ecc5487798..677589e3e35 100644
--- a/cabal-install/src/Distribution/Client/CmdHaddock.hs
+++ b/cabal-install/src/Distribution/Client/CmdHaddock.hs
@@ -136,7 +136,8 @@ mkFlagsAbsolute :: NixStyleFlags ClientHaddockFlags -> IO (NixStyleFlags ClientH
 mkFlagsAbsolute relFlags = do
   let relHaddockFlags = haddockFlags relFlags
   absHaddockOutputDir <- traverse makeAbsolute (haddockOutputDir relHaddockFlags)
-  return (relFlags{haddockFlags = relHaddockFlags{haddockOutputDir = absHaddockOutputDir}})
+  absHaddockCss <- traverse makeAbsolute (haddockCss relHaddockFlags)
+  return (relFlags{haddockFlags = relHaddockFlags{haddockOutputDir = absHaddockOutputDir, haddockCss = absHaddockCss}})
 
 -- | The @haddock@ command is TODO.
 --