handle undefined __kbnThemeTag__ globals caused by test runners

packages/kbn-ui-shared-deps/theme.ts

@@ -23,8 +23,11 @@ const globals: any = typeof window === 'undefined' ? {} : window;
 
 export type Theme = typeof LightTheme;
 
-export const version: 7 | 8 = globals.__kbnThemeTag__.startsWith('v7') ? 7 : 8;
-export const darkMode: boolean = globals.__kbnThemeTag__.endsWith('dark');
+// in the Kibana app we can rely on this global being defined, but in
+// some cases (like jest, or karma tests) the global is undefined
+export const tag: string = globals.__kbnThemeTag__ || 'v7light';
+export const version = tag.startsWith('v7') ? 7 : 8;
+export const darkMode = tag.endsWith('dark');
 
 export let euiLightVars: Theme;
 export let euiDarkVars: Theme;