diff --git a/external/builder/builder.js b/external/builder/builder.js
index f0b3d53b7..378de9bd6 100644
--- a/external/builder/builder.js
+++ b/external/builder/builder.js
@@ -199,7 +199,7 @@ function preprocess(inFilename, outFilename, defines) {
 }
 exports.preprocess = preprocess;
 
-function preprocessCSS(mode, source, destination) {
+function preprocessCSS(inFilename, outFilename, defines) {
   function hasPrefixedMozcentral(line) {
     return /(^|\W)-(ms|o|webkit)-\w/.test(line);
   }
@@ -262,16 +262,16 @@ function preprocessCSS(mode, source, destination) {
     return lines.join("\n");
   }
 
-  if (!mode) {
-    throw new Error("Invalid CSS preprocessor mode");
+  if (!defines) {
+    throw new Error("Missing CSS preprocessor defines.");
   }
 
-  let content = fs.readFileSync(source, "utf8").toString();
-  content = expandImports(content, source);
-  if (mode === "mozcentral") {
+  let content = fs.readFileSync(inFilename, "utf8").toString();
+  content = expandImports(content, inFilename);
+  if (defines.MOZCENTRAL) {
     content = removePrefixed(content, hasPrefixedMozcentral);
   }
-  fs.writeFileSync(destination, content);
+  fs.writeFileSync(outFilename, content);
 }
 exports.preprocessCSS = preprocessCSS;
 
diff --git a/gulpfile.js b/gulpfile.js
index 4e2688293..bcb0027dc 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -801,9 +801,9 @@ gulp.task("cmaps", function (done) {
   done();
 });
 
-function preprocessCSS(source, mode, defines) {
+function preprocessCSS(source, defines) {
   const outName = getTempFile("~preprocess", ".css");
-  builder.preprocessCSS(mode, source, outName);
+  builder.preprocessCSS(source, outName, defines);
   let out = fs.readFileSync(outName).toString();
   fs.unlinkSync(outName);
 
@@ -848,7 +848,7 @@ function buildGeneric(defines, dir) {
     createStandardFontBundle().pipe(gulp.dest(dir + "web/standard_fonts")),
 
     preprocessHTML("web/viewer.html", defines).pipe(gulp.dest(dir + "web")),
-    preprocessCSS("web/viewer.css", "generic", defines)
+    preprocessCSS("web/viewer.css", defines)
       .pipe(postcss([calc(), autoprefixer(AUTOPREFIXER_CONFIG)]))
       .pipe(gulp.dest(dir + "web")),
 
@@ -924,7 +924,7 @@ function buildComponents(defines, dir) {
   return merge([
     createComponentsBundle(defines).pipe(gulp.dest(dir)),
     gulp.src(COMPONENTS_IMAGES).pipe(gulp.dest(dir + "images")),
-    preprocessCSS("web/pdf_viewer.css", "components", defines)
+    preprocessCSS("web/pdf_viewer.css", defines)
       .pipe(postcss([calc(), autoprefixer(AUTOPREFIXER_CONFIG)]))
       .pipe(gulp.dest(dir)),
   ]);
@@ -1014,7 +1014,7 @@ function buildMinified(defines, dir) {
     createStandardFontBundle().pipe(gulp.dest(dir + "web/standard_fonts")),
 
     preprocessHTML("web/viewer.html", defines).pipe(gulp.dest(dir + "web")),
-    preprocessCSS("web/viewer.css", "minified", defines)
+    preprocessCSS("web/viewer.css", defines)
       .pipe(postcss([calc(), autoprefixer(AUTOPREFIXER_CONFIG)]))
       .pipe(gulp.dest(dir + "web")),
 
@@ -1244,7 +1244,7 @@ gulp.task(
         preprocessHTML("web/viewer.html", defines).pipe(
           gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")
         ),
-        preprocessCSS("web/viewer.css", "mozcentral", defines)
+        preprocessCSS("web/viewer.css", defines)
           .pipe(
             postcss([
               autoprefixer({
@@ -1336,7 +1336,7 @@ gulp.task(
         preprocessHTML("web/viewer.html", defines).pipe(
           gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")
         ),
-        preprocessCSS("web/viewer.css", "chrome", defines)
+        preprocessCSS("web/viewer.css", defines)
           .pipe(
             postcss([autoprefixer({ overrideBrowserslist: ["Chrome >= 73"] })])
           )
@@ -1871,7 +1871,7 @@ gulp.task("dev-css", function createDevCSS() {
   return merge([
     gulp.src("web/images/*", { base: "web/" }).pipe(gulp.dest(cssDir)),
 
-    preprocessCSS("web/viewer.css", "generic", defines)
+    preprocessCSS("web/viewer.css", defines)
       .pipe(
         postcss([autoprefixer({ overrideBrowserslist: ["last 2 versions"] })])
       )