diff --git a/core/frontend/services/theme-engine/middleware.js b/core/frontend/services/theme-engine/middleware.js
index 375ac844c7..cd7fd5ce2e 100644
--- a/core/frontend/services/theme-engine/middleware.js
+++ b/core/frontend/services/theme-engine/middleware.js
@@ -87,12 +87,9 @@ async function getProductAndPricesData() {
     }
 }
 
-function getSiteData(req) {
+function getSiteData() {
     let siteData = settingsCache.getPublic();
 
-    // @TODO: it would be nicer if this was proper middleware somehow...
-    siteData = preview.handle(req, siteData);
-
     // theme-only computed property added to @site
     if (settingsCache.get('members_signup_access') === 'none') {
         const escapedUrl = encodeURIComponent(urlUtils.urlFor({relativeUrl: '/rss/'}, true));
@@ -108,7 +105,7 @@ async function updateGlobalTemplateOptions(req, res, next) {
     // Static information, same for every request unless the settings change
     // @TODO: bind this once and then update based on events?
     // @TODO: decouple theme layer from settings cache using the Content API
-    const siteData = getSiteData(req);
+    const siteData = getSiteData();
     const labsData = labs.getAll();
 
     const themeData = {
@@ -157,10 +154,18 @@ function updateLocalTemplateData(req, res, next) {
 
 function updateLocalTemplateOptions(req, res, next) {
     const localTemplateOptions = hbs.getLocalTemplateOptions(res.locals);
+
+    // adjust @site.url for http/https based on the incoming request
     const siteData = {
         url: urlUtils.urlFor('home', {secure: req.secure, trailingSlash: false}, true)
     };
 
+    // @TODO: it would be nicer if this was proper middleware somehow...
+    const previewData = preview.handle(req);
+
+    // update site data with any preview values from the request
+    Object.assign(siteData, previewData);
+
     const member = req.member ? {
         uuid: req.member.uuid,
         email: req.member.email,
diff --git a/core/frontend/services/theme-engine/preview.js b/core/frontend/services/theme-engine/preview.js
index e42b479a16..76c8d06332 100644
--- a/core/frontend/services/theme-engine/preview.js
+++ b/core/frontend/services/theme-engine/preview.js
@@ -14,7 +14,7 @@ function decodeValue(value) {
     return value;
 }
 
-function getPreviewData(previewHeader, siteData) {
+function getPreviewData(previewHeader) {
     // Keep the string shorter with short codes for certain parameters
     const supportedSettings = {
         c: 'accent_color',
@@ -25,23 +25,27 @@ function getPreviewData(previewHeader, siteData) {
 
     let opts = new URLSearchParams(previewHeader);
 
+    const previewData = {};
+
     opts.forEach((value, key) => {
         value = decodeValue(value);
         if (supportedSettings[key]) {
-            _.set(siteData, supportedSettings[key], value);
+            _.set(previewData, supportedSettings[key], value);
         }
     });
 
-    siteData._preview = previewHeader;
+    previewData._preview = previewHeader;
 
-    return siteData;
+    return previewData;
 }
 
 module.exports._PREVIEW_HEADER_NAME = PREVIEW_HEADER_NAME;
-module.exports.handle = (req, siteData) => {
+module.exports.handle = (req) => {
+    let previewData = {};
+
     if (req && req.header(PREVIEW_HEADER_NAME)) {
-        siteData = getPreviewData(req.header(PREVIEW_HEADER_NAME), siteData);
+        previewData = getPreviewData(req.header(PREVIEW_HEADER_NAME));
     }
 
-    return siteData;
+    return previewData;
 };
diff --git a/test/unit/services/theme-engine/middleware.test.js b/test/unit/services/theme-engine/middleware.test.js
index 0f06f86dde..a239183c40 100644
--- a/test/unit/services/theme-engine/middleware.test.js
+++ b/test/unit/services/theme-engine/middleware.test.js
@@ -66,6 +66,7 @@ describe('Themes middleware', function () {
             .returns(fakeSiteData);
 
         sandbox.stub(hbs, 'updateTemplateOptions');
+        sandbox.stub(hbs, 'updateLocalTemplateOptions');
     });
 
     it('mounts active theme if not yet mounted', function (done) {
@@ -194,7 +195,7 @@ describe('Themes middleware', function () {
     });
 
     describe('Preview Mode', function () {
-        it('calls updateTemplateOptions with correct data when one parameter is set', function (done) {
+        it('calls updateLocalTemplateOptions with correct data when one parameter is set', function (done) {
             const previewString = 'c=%23000fff';
             req.header = () => {
                 return previewString;
@@ -204,13 +205,11 @@ describe('Themes middleware', function () {
                 try {
                     should.not.exist(err);
 
-                    hbs.updateTemplateOptions.calledOnce.should.be.true();
-                    const templateOptions = hbs.updateTemplateOptions.firstCall.args[0];
+                    hbs.updateLocalTemplateOptions.calledOnce.should.be.true();
+                    const templateOptions = hbs.updateLocalTemplateOptions.firstCall.args[1];
                     const data = templateOptions.data;
 
-                    data.should.be.an.Object().with.properties('site', 'labs', 'config');
-
-                    should.equal(data.site, fakeSiteData);
+                    data.should.be.an.Object().with.properties('site');
 
                     data.site.should.be.an.Object().with.properties('accent_color', '_preview');
                     data.site._preview.should.eql(previewString);
@@ -223,7 +222,7 @@ describe('Themes middleware', function () {
             });
         });
 
-        it('calls updateTemplateOptions with correct data when two parameters are set', function (done) {
+        it('calls updateLocalTemplateOptions with correct data when two parameters are set', function (done) {
             const previewString = 'c=%23000fff&icon=%2Fcontent%2Fimages%2Fmyimg.png';
             req.header = () => {
                 return previewString;
@@ -233,13 +232,11 @@ describe('Themes middleware', function () {
                 try {
                     should.not.exist(err);
 
-                    hbs.updateTemplateOptions.calledOnce.should.be.true();
-                    const templateOptions = hbs.updateTemplateOptions.firstCall.args[0];
+                    hbs.updateLocalTemplateOptions.calledOnce.should.be.true();
+                    const templateOptions = hbs.updateLocalTemplateOptions.firstCall.args[1];
                     const data = templateOptions.data;
 
-                    data.should.be.an.Object().with.properties('site', 'labs', 'config');
-
-                    should.equal(data.site, fakeSiteData);
+                    data.should.be.an.Object().with.properties('site');
 
                     data.site.should.be.an.Object().with.properties('accent_color', 'icon', '_preview');
                     data.site._preview.should.eql(previewString);