Update README with theming documentation and examples

- Add Theming section explaining --theme CLI option
- Add Examples section with links to all generated PDFs
- Include link to colorful test theme as reference
- Document custom CSS styling capabilities

Author: rohaanhamid

README.md

@@ -59,6 +59,31 @@ You can control which pages from an embedded PDF are included using simple opera
 
 You cannot specify both `!=` and `=` for the same PDF (the tool will throw an error).
 
+## Theming
+
+Customize the appearance of your generated PDFs by providing a custom CSS file:
+
+```bash
+document-stitcher input.md output.pdf --theme custom-theme.css
+```
+
+The CSS file will be applied to the HTML before PDF generation, giving you full control over fonts, colors, spacing, and layout. See [test_theme.scss](tests/fixtures/theme/test_theme.scss) for an example of a colorful theme.
+
+## Examples
+
+Here are some example PDFs generated from the test fixtures:
+
+- [Basic Markdown](test-output/integration/basic-generated.pdf) - Simple markdown rendering
+- [Combined Features](test-output/integration/combined-generated.pdf) - Multiple markdown features
+- [Embedded PDFs](test-output/integration/embed-pdf-generated.pdf) - PDF embedding functionality
+- [Images](test-output/integration/images-generated.pdf) - Image inclusion and sizing
+- [Math Expressions](test-output/integration/math-generated.pdf) - LaTeX math rendering
+- [Multi-page PDFs](test-output/integration/multi_page_full-generated.pdf) - Full PDF embedding
+- [Page Selection](test-output/integration/multi_page_include-generated.pdf) - Selective page inclusion
+- [Page Skipping](test-output/integration/multi_page_skip-generated.pdf) - Page exclusion
+- [Page Breaks](test-output/integration/pagebreaks-generated.pdf) - Manual page breaks
+- [Custom Theme](test-output/integration/themed-generated.pdf) - Colorful themed output
+
 ## License
 
 This project is licensed under the ISC License - see the [LICENSE](LICENSE) file for details.