style(chat): smaller headers, fix no-padding

[ctlai95]

Problem

• Markdown headers are too large
• no-padding from mynah-ui is not working

Before:

Solution

• Override the font size for h1-h4
• Override the padding for any card under no-padding

After:

---

• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Hweinstock]

I imagine this fix should ideally live in Mynah since this fix will be needed for all consumers of Mynah right?