Adding brand.yml support

[durraniu]

Description

I switched theme with brand: true in survey.qmd and it largely applied the provided _brand.yml settings. The primary and foreground colors were applied but not the background color. However, setting theme: vapor does apply the complete theme colors including the background color.
It'd be great if the brand.yml is completely supported. Thanks.

[jhelvy]

We've now released v1.0.0 if you wanted to look into this. I believe it will be related to #233 as it will involve fixing how we pass all of the Quarto settings (JS, CSS, etc) through to the resulting shiny app.