diff --git a/packages/base/cypress/specs/Boot.cy.tsx b/packages/base/cypress/specs/Boot.cy.tsx
index d13bc8a1a81b..4318b72308c3 100644
--- a/packages/base/cypress/specs/Boot.cy.tsx
+++ b/packages/base/cypress/specs/Boot.cy.tsx
@@ -16,4 +16,50 @@ describe("Framework boot", () => {
 			.invoke("hasStyle", "data-ui5-theme-properties", "@ui5/webcomponents-theming")
 			.should("be.true");
 	});
+
+	it("Tests theme loading, when registered after boot + theme props are registered for the base theme of an external theme", () => {
+		const baseTheme = "sap_horizon"
+		const customTheme = "my_custom_theme";
+		const customThemeStyles = `.sapThemeMetaData-Base-baseLib{background-image: url('data:text/plain;utf-8,{"Path": "Base.baseLib.${customTheme}.css_variables", "PathPattern": "/%frameworkId%/%libId%/%themeId%/%fileId%.css", "Extends": ["${baseTheme}"], "Tags": ["Fiori_3","LightColorScheme"], "FallbackThemeId": "${baseTheme}", "Engine": {"Name": "theming-engine", "Version": "16.1.10"}, "Version": { "Build": "11.32.2.20251030081249", "Source": "11.32.2"}}');}`;
+		const dataPropAttr = `data-ui5-component-properties-0`;
+
+		// Append meta style and config script to the head setting the custom theme
+		cy.window()
+			.then($el => {
+				const style = document.createElement("style");
+				style.innerHTML = customThemeStyles;
+				return $el.document.head.append(style);
+			})
+			.then($el => {
+				const scriptElement = document.createElement("script");
+				scriptElement.type = "application/json";
+				scriptElement.setAttribute("data-ui5-config", "true");
+				scriptElement.innerHTML = JSON.stringify({
+					"theme": customTheme,
+				});
+				return $el.document.head.append(scriptElement);
+			});
+
+
+		// Trigger boot
+		cy.wrap({ boot })
+			.invoke("boot");
+
+		// After boot, register theme properties loader for the base theme
+		cy.wrap({ registerThemePropertiesLoader })
+			.invoke("registerThemePropertiesLoader", "@ui5/webcomponents-test", baseTheme, () => {
+				return Promise.resolve(`:root{--_ui5_internal_var: #ccc }`);
+			});
+
+		// Assert - theme properties for the base theme are loaded
+		cy.document().should(($doc) => {
+			const adoptedStyleSheets = $doc.adoptedStyleSheets;
+			const sheet = adoptedStyleSheets.find(sh => (sh as any)._ui5StyleId === `${dataPropAttr}|@ui5/webcomponents-test`);
+
+			expect(sheet, "stylesheet should exist").to.exist;
+			expect(sheet!.cssRules, "cssRules should exist").to.exist;
+			expect(sheet!.cssRules.length, "cssRules should have at least one rule").to.be.greaterThan(0);
+			expect(sheet!.cssRules[0].cssText, "the custom variable should exist").to.include("--_ui5_internal_var: #ccc");
+		});
+	});
 });
diff --git a/packages/base/src/Boot.ts b/packages/base/src/Boot.ts
index dd03ed99a2b3..e5259a9cd8ed 100644
--- a/packages/base/src/Boot.ts
+++ b/packages/base/src/Boot.ts
@@ -3,7 +3,7 @@ import EventProvider from "./EventProvider.js";
 import insertFontFace from "./FontFace.js";
 import insertSystemCSSVars from "./SystemCSSVars.js";
 import insertScrollbarStyles from "./ScrollbarStyles.js";
-import { getTheme } from "./config/Theme.js";
+import { getTheme, getBaseTheme } from "./config/Theme.js";
 import applyTheme from "./theming/applyTheme.js";
 import { registerCurrentRuntime } from "./Runtimes.js";
 import { getFeature } from "./FeaturesRegistry.js";
@@ -88,8 +88,15 @@ const boot = async (): Promise<void> => {
  * @param { string } theme
  */
 const onThemeRegistered = (theme: string) => {
-	if (booted && theme === getTheme()) { // getTheme should only be called if "booted" is true
-		applyTheme(getTheme());
+	if (!booted) {
+		return;
+	}
+
+	const currentTheme = getTheme();
+	const currentBaseTheme = getBaseTheme();
+
+	if (theme === currentTheme || theme === currentBaseTheme) {
+		applyTheme(currentTheme);
 	}
 };
 
diff --git a/packages/base/src/config/Theme.ts b/packages/base/src/config/Theme.ts
index 4f5791796a05..0cea7adcb994 100644
--- a/packages/base/src/config/Theme.ts
+++ b/packages/base/src/config/Theme.ts
@@ -7,6 +7,7 @@ import { boot, isBooted } from "../Boot.js";
 import { attachConfigurationReset } from "./ConfigurationReset.js";
 
 let curTheme: string | undefined;
+let curBaseTheme: string | undefined;
 
 attachConfigurationReset(() => {
 	curTheme = undefined;
@@ -91,6 +92,24 @@ const isLegacyThemeFamilyAsync = async () => {
 
 const isKnownTheme = (theme: string) => SUPPORTED_THEMES.includes(theme);
 
+/**
+ * Returns the base theme of external theme.
+ * @private
+ * @returns {string | undefined} the base theme name
+ */
+const getBaseTheme = (): string | undefined => {
+	return curBaseTheme;
+};
+
+/**
+ * Sets the base theme of the current external theme.
+ * @param { string | undefined } theme the name of the new base theme
+ * @private
+ */
+const setBaseTheme = (theme: string | undefined): void => {
+	curBaseTheme = theme;
+};
+
 export {
 	getTheme,
 	setTheme,
@@ -98,4 +117,6 @@ export {
 	isLegacyThemeFamily,
 	isLegacyThemeFamilyAsync,
 	getDefaultTheme,
+	getBaseTheme,
+	setBaseTheme,
 };
diff --git a/packages/base/src/theming/applyTheme.ts b/packages/base/src/theming/applyTheme.ts
index c8f7d729c156..9ed487f67b25 100644
--- a/packages/base/src/theming/applyTheme.ts
+++ b/packages/base/src/theming/applyTheme.ts
@@ -4,6 +4,7 @@ import getThemeDesignerTheme from "./getThemeDesignerTheme.js";
 import { fireThemeLoaded } from "./ThemeLoaded.js";
 import { getFeature } from "../FeaturesRegistry.js";
 import { attachCustomThemeStylesToHead, getThemeRoot } from "../config/ThemeRoot.js";
+import { setBaseTheme } from "../config/Theme.js";
 import type OpenUI5Support from "../features/OpenUI5Support.js";
 import { DEFAULT_THEME } from "../generated/AssetParameters.js";
 import { getCurrentRuntimeIndex } from "../Runtimes.js";
@@ -87,9 +88,13 @@ const applyTheme = async (theme: string) => {
 	}
 
 	// Always load component packages properties. For non-registered themes, try with the base theme, if any
-	const packagesTheme = isThemeRegistered(theme) ? theme : extTheme && extTheme.baseThemeName;
-	await loadComponentPackages(packagesTheme || DEFAULT_THEME, extTheme && extTheme.themeName === theme ? theme : undefined);
+	const externalThemeName = extTheme && extTheme.themeName === theme ? theme : undefined;
+	const baseThemeName = extTheme && extTheme.baseThemeName;
+	const effectiveThemeName = isThemeRegistered(theme) ? theme : (baseThemeName || DEFAULT_THEME);
 
+	await loadComponentPackages(effectiveThemeName, externalThemeName);
+
+	setBaseTheme(baseThemeName);
 	fireThemeLoaded(theme);
 };