diff --git a/Tests/AppKit/CPThemeDataTest.j b/Tests/AppKit/CPThemeDataTest.j
new file mode 100644
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+++ b/Tests/AppKit/CPThemeDataTest.j
@@ -0,0 +1,196 @@
+/*
+ *  CPThemeDataTest.j
+ *  AppKit
+ *
+ *  Created by David Richardson on 2026-11-03.
+ *  Copyright David Richardson 2026.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+@import <AppKit/AppKit.j>
+
+/*
+    A theme holds values: colours, images, metrics, fonts and flags. It holds no
+    code. Behaviour, such as animation, event handling and layout logic, stays in
+    the class, where an application can override it, so that a change of theme
+    changes appearance and never behaviour.
+
+    These tests read every theme in the built blends and fail on any value that is
+    a function, or a string holding a function's source. They read the build, so
+    they need the themes built first, as `npx jake build` does.
+*/
+var fs = require("fs"),
+    path = require("path");
+
+var CPThemeDataTestCodePattern = /^\s*\(?\s*(async\s+)?function\b|^\s*\(?[\w\s,]*\)?\s*=>/;
+
+@implementation CPThemeDataTest : OJTestCase
+{
+}
+
+- (void)setUp
+{
+    // This will init the global var CPApp which are used internally in the AppKit
+    [[CPApplication alloc] init];
+}
+
+/*
+    Returns the directory of the built blends: Debug if it exists, otherwise Release.
+*/
+- (CPString)buildDirectory
+{
+    var buildPath = process.env.CAPP_BUILD || process.env.BUILD_PATH || "Build",
+        configurations = ["Debug", "Release"];
+
+    for (var i = 0; i < configurations.length; i++)
+    {
+        var directory = path.resolve(buildPath, configurations[i]);
+
+        if (fs.existsSync(directory) && fs.readdirSync(directory).some(function(aName) { return /\.blend$/.test(aName); }))
+            return directory;
+    }
+
+    return nil;
+}
+
+/*
+    Returns the keyed theme archives in a blend's static file, as an array of
+    { path, text } objects. A static file is a sequence of records, each a type
+    letter, a length and that many characters: a "p" record names a file, and the
+    "t" record after it holds the file's text.
+*/
+- (CPArray)keyedThemesInStaticFile:(CPString)aStaticFile
+{
+    var contents = fs.readFileSync(aStaticFile, { encoding: "utf8" }),
+        themes = [],
+        index = contents.indexOf(";", contents.indexOf(";") + 1) + 1,
+        currentPath = nil;
+
+    while (index < contents.length)
+    {
+        var type = contents.charAt(index),
+            lengthEnd = contents.indexOf(";", index + 2),
+            length = parseInt(contents.substring(index + 2, lengthEnd), 10),
+            text = contents.substr(lengthEnd + 1, length);
+
+        if (type === "p")
+            currentPath = text;
+        else if (type === "t" && currentPath && /\.keyedtheme$/.test(currentPath))
+            themes.push({ path: currentPath, text: text });
+
+        index = lengthEnd + 1 + length;
+    }
+
+    return themes;
+}
+
+/*
+    Adds a description of every code value inside aValue to someViolations. Arrays
+    and dictionaries, such as CSS dictionaries, are searched.
+*/
+- (void)collectCodeIn:(id)aValue at:(CPString)aLocation into:(CPArray)someViolations
+{
+    if (aValue === nil || aValue === undefined)
+        return;
+
+    if (typeof aValue === "function")
+    {
+        someViolations.push(aLocation + " is a function");
+        return;
+    }
+
+    if (typeof aValue === "string")
+    {
+        if (CPThemeDataTestCodePattern.test(aValue))
+            someViolations.push(aLocation + " is a string holding a function's source");
+
+        return;
+    }
+
+    if (!aValue.isa)
+        return;
+
+    if ([aValue isKindOfClass:[CPArray class]])
+    {
+        for (var i = 0; i < aValue.length; i++)
+            [self collectCodeIn:aValue[i] at:aLocation + "[" + i + "]" into:someViolations];
+    }
+    else if ([aValue isKindOfClass:[CPDictionary class]])
+    {
+        var keys = [aValue allKeys];
+
+        for (var i = 0; i < keys.length; i++)
+            [self collectCodeIn:[aValue objectForKey:keys[i]] at:aLocation + "." + keys[i] into:someViolations];
+    }
+}
+
+- (void)testThemeValuesAreData
+{
+    var buildDirectory = [self buildDirectory];
+
+    [self assertTrue:buildDirectory !== nil message:@"No built blends were found. Build the themes first."];
+
+    var blends = fs.readdirSync(buildDirectory).filter(function(aName) { return /\.blend$/.test(aName); }).sort(),
+        themeCount = 0,
+        violations = [];
+
+    for (var i = 0; i < blends.length; i++)
+    {
+        var blendPath = path.join(buildDirectory, blends[i]),
+            staticFile = path.join(blendPath, "CommonJS.environment", blends[i] + ".sj");
+
+        if (!fs.existsSync(staticFile))
+            continue;
+
+        var bundle = [CPBundle bundleWithPath:blendPath],
+            keyedThemes = [self keyedThemesInStaticFile:staticFile];
+
+        for (var j = 0; j < keyedThemes.length; j++)
+        {
+            var unarchiver = [[_CPThemeKeyedUnarchiver alloc] initForReadingWithData:[CPData dataWithRawString:keyedThemes[j].text] bundle:bundle],
+                theme = [unarchiver decodeObjectForKey:@"root"];
+
+            [unarchiver finishDecoding];
+            themeCount++;
+
+            var classNames = [[theme classNames] sortedArrayUsingSelector:@selector(compare:)];
+
+            for (var k = 0; k < classNames.length; k++)
+            {
+                var attributes = [theme attributesForClass:classNames[k]],
+                    names = [[attributes allKeys] sortedArrayUsingSelector:@selector(compare:)];
+
+                for (var m = 0; m < names.length; m++)
+                {
+                    var attribute = [attributes objectForKey:names[m]],
+                        location = [theme name] + " " + classNames[k] + " " + names[m],
+                        values = [attribute values],
+                        states = [values allKeys];
+
+                    [self collectCodeIn:[attribute defaultValue] at:location + " (default)" into:violations];
+
+                    for (var n = 0; n < states.length; n++)
+                        [self collectCodeIn:[values objectForKey:states[n]] at:location + " (" + states[n] + ")" into:violations];
+                }
+            }
+        }
+    }
+
+    [self assertTrue:themeCount > 0 message:@"No themes were found in the built blends."];
+    [self assertTrue:violations.length === 0 message:@"Theme values must be data, not code:\n" + violations.join("\n")];
+}
+
+@end