diff --git a/Cabal/src/Distribution/Simple/Haddock.hs b/Cabal/src/Distribution/Simple/Haddock.hs
index 01bc17828e2..ee2b88e76af 100644
--- a/Cabal/src/Distribution/Simple/Haddock.hs
+++ b/Cabal/src/Distribution/Simple/Haddock.hs
@@ -617,6 +617,7 @@ 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.
 --
diff --git a/changelog.d/pr-10637 b/changelog.d/pr-10637
new file mode 100644
index 00000000000..0cff281a4d0
--- /dev/null
+++ b/changelog.d/pr-10637
@@ -0,0 +1,11 @@
+---
+synopsis: Fix Haddock CSS handling in multi-package projects
+packages: [cabal-install, Cabal]
+prs: 10637
+issues: [10636]
+---
+
+When `--css=<css-file>` flag is provided to `cabal haddock-project`:
+
+- the Haddock index is now properly styled by the provided CSS file
+- each package in the project now has their docs properly styled by the provided CSS file