Short answer
HubSpot's KB: set default fonts, colors and layout in Customize quote style; template-level branding overrides it. After a theme is saved, later brand kit changes don't apply. Assigning a Revenue Hub seat removes access to legacy quotes.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Update the brand kit before saving a quote theme.
3. Related
Quote basics and brand kit.