theme: add support for setting default theme via global variable

packages/theme/src/index.ts

@@ -18,7 +18,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 import {
   SchemeColors,
-  SchemeColorsAsCSSVariables
+  SchemeColorsAsCSSVariables,
+  ThemeDefinition
 } from "./theme-engine/types.js";
 
 export * from "./theme/index.js";
@@ -26,6 +27,11 @@ export * from "./theme-engine/index.js";
 export * from "./theme-engine/types.js";
 export * from "./emotion/index.js";
 
+declare global {
+  // eslint-disable-next-line no-var
+  var DEFAULT_THEME: ThemeDefinition | undefined;
+}
+
 declare module "csstype" {
   interface Properties {
     backgroundColor?:

packages/theme/src/theme-engine/index.ts

@@ -43,7 +43,7 @@ type ThemeEngineState = {
   setTheme: (theme: ThemeDefinition) => void;
 };
 const useThemeEngineStore = create<ThemeEngineState>((set) => ({
-  theme: ThemeLight,
+  theme: globalThis.DEFAULT_THEME || ThemeLight,
   setTheme: (theme) => set({ theme })
 }));