Nbconvert

- HTMLExporter: Respect the embed_images flag for HTML blocks
[1721](https://github.com/jupyter/nbconvert/pull/1721)

- Remove ipython genutils [1727](https://github.com/jupyter/nbconvert/pull/1727)
- Add section to customizing showing how to use template inheritance
[1719](https://github.com/jupyter/nbconvert/pull/1719)

- Adding theme support for WebPDF exporter [1718](https://github.com/jupyter/nbconvert/pull/1718)
- Add option to embed_images in Markdown cells
[1717](https://github.com/jupyter/nbconvert/pull/1717)
- HTMLExporter: Add theme alias and docs [1716](https://github.com/jupyter/nbconvert/pull/1716)
- Add basic support for federated labextensions themes
[1703](https://github.com/jupyter/nbconvert/pull/1703)
- Always hide the collapser element [1712](https://github.com/jupyter/nbconvert/pull/1712)
- Raise pyppeteer requirement to >=1,\<1.1 [1711](https://github.com/jupyter/nbconvert/pull/1711)

- Handle needs_background cell metadata [1704](https://github.com/jupyter/nbconvert/pull/1704)
- Fix styling regression [1708](https://github.com/jupyter/nbconvert/pull/1708)
- Fix DOM structure of markdown cells in lab template
[1709](https://github.com/jupyter/nbconvert/pull/1709)
- CodeMirror style bleed fix [1710](https://github.com/jupyter/nbconvert/pull/1710)

The full list of changes can be seen on the [6.4.0
milestone](https://github.com/jupyter/nbconvert/milestone/23?closed=1)

- Allow passing extra args to code highlighter
[1683](https://github.com/jupyter/nbconvert/pull/1683)
- Prevent page breaks in outputs when printing
[1679](https://github.com/jupyter/nbconvert/pull/1679)
- Add collapsers to template [1689](https://github.com/jupyter/nbconvert/pull/1689)
- Optionally speed up validation [1672](https://github.com/jupyter/nbconvert/pull/1672)

The full list of changes can be seen on the [6.3.0
milestone](https://github.com/jupyter/nbconvert/milestone/22?closed=1)

- Update state filter [1664](https://github.com/jupyter/nbconvert/pull/1664)
- Add slide numbering [1654](https://github.com/jupyter/nbconvert/pull/1654)
- Fix HTML templates mentioned in help docs [1653](https://github.com/jupyter/nbconvert/pull/1653)