feat: expose theme enum and store to consumer (#578)

# Motivation

Some consumers, such as OISY, might want to implement their own theme
toggler. Therefore to avoid conflicts with different themes, we expose
the enum and store of the library for consumers as well (as we do with
other stores).

# Changes

- Expose enum `Theme` and `theme.store`

src/lib/index.ts

@@ -5,11 +5,13 @@ export * from "./icons";
 export * from "./stores/busy.store";
 export * from "./stores/layout.store";
 export * from "./stores/menu.store";
+export * from "./stores/theme.store";
 export * from "./stores/toasts.store";
 export * from "./stores/wizard.state";
 export type { BusyState } from "./types/busy";
 export type { ProgressBarSegment } from "./types/progress-bar";
 export type { ProgressStep, ProgressStepState } from "./types/progress-step";
+export * from "./types/theme";
 export type { ToastLevel, ToastMsg } from "./types/toast";
 export type { WizardStep, WizardSteps } from "./types/wizard";
 export * from "./utils/wizard.utils";