Fix exports for CSSClasses and ThemeOptions

[niloysikdar]

Description:

While creating one demo application and layout using our package in TypeScript, I realized that we need CSSClasses and ThemeOptions interfaces to declare types for the variables.

Solution:

We need to export these interfaces directly from the package so that users can use them.

[github-actions]

🎉 This PR is included in version 1.15.2 🎉