diff --git a/packages/shared-core/src/tests/themes.ts b/packages/shared-core/src/tests/themes.ts
new file mode 100644
index 0000000000..89c6910ad5
--- /dev/null
+++ b/packages/shared-core/src/tests/themes.ts
@@ -0,0 +1,28 @@
+import { getThemeClassNames, ensureValidTheme } from "../themes"
+import { Theme } from "@budibase/types"
+
+describe("theme class names", () => {
+  it("generates class names for a theme without base theme", () => {
+    expect(getThemeClassNames(Theme.LIGHT)).toStrictEqual("spectrum--light")
+  })
+  it("generates class names for a theme with base theme", () => {
+    expect(getThemeClassNames(Theme.NORD)).toStrictEqual(
+      "spectrum--darkest spectrum--nord"
+    )
+  })
+})
+
+describe("theme validity checking", () => {
+  it("handles no theme", () => {
+    expect(ensureValidTheme(undefined)).toStrictEqual(Theme.DARKEST)
+  })
+  it("allows specifiying a fallback", () => {
+    expect(ensureValidTheme(undefined, Theme.NORD)).toStrictEqual(Theme.NORD)
+  })
+  it("migrates lightest to light", () => {
+    expect(ensureValidTheme(Theme.LIGHTEST)).toStrictEqual(Theme.LIGHT)
+  })
+  it("migrates dark to darkest", () => {
+    expect(ensureValidTheme(Theme.DARK)).toStrictEqual(Theme.DARKEST)
+  })
+})
diff --git a/packages/shared-core/src/themes.ts b/packages/shared-core/src/themes.ts
index 1d1875d4e9..d70f188426 100644
--- a/packages/shared-core/src/themes.ts
+++ b/packages/shared-core/src/themes.ts
@@ -17,7 +17,7 @@ export const getThemeClassNames = (theme: Theme): string => {
 
 // Ensures a theme value is a valid option
 export const ensureValidTheme = (
-  theme: Theme,
+  theme?: Theme,
   fallback: Theme = Theme.DARKEST
 ): Theme => {
   if (!theme) {