[7.4] Story: Create Theming Documentation

## Description

Document how to customize component styling.

**Parent Issue:** #EPIC_7

### Sub-Tasks
- [ ] Document CSS custom properties
- [ ] Write NCIDS theme integration guide
- [ ] Explain custom theme creation
- [ ] Document component-specific overrides
- [ ] Add common customization examples

### Notes
* Theming enables consistent branding across apps