feat(striker-ui): add default theme

striker-ui/lib/consts/DEFAULT_THEME.ts

@@ -0,0 +1,10 @@
+import { DefaultTheme } from 'styled-components';
+
+const DEFAULT_THEME: Readonly<DefaultTheme> = {
+  colors: {
+    primary: '#f2f2f2',
+    secondary: '#343434',
+  },
+};
+
+export default DEFAULT_THEME;

striker-ui/types/DefaultTheme.d.ts

@@ -0,0 +1,10 @@
+import 'styled-components';
+
+declare module 'styled-components' {
+  export interface DefaultTheme {
+    colors: {
+      primary: string;
+      secondary: string;
+    };
+  }
+}