Pattern Previews: Use less-opinionated base theme

[ryelle]

Fixes #143 - Switch the pattern preview theme styles to use Twenty Twenty-One with a white background as the base CSS. This also removes some other CSS dependencies that are not necessary in this context (non-editable). This brings the number of files loaded per each iframe down to two - wp-block-library and the theme styles (see #255).

Screenshots

How to test the changes in this Pull Request:

1. Check out branch and build with yarn workspaces run build, or just look at the screenshots ^
2. View the pattern previews in grid mode
3. They should look as expected - headings are plain sans-serif, no top border. Body text is also sans-serif. Background should be white (unless the pattern uses a background).
4. View single pattern previews
5. They should look as expected, see above

[StevenDufresne]

Speed looks better on load! Good call.

Are we going to assume that some CSS styles will not work or conflict?

The button is background-color: #fff on color: #fff:

/pattern/introduction-with-image-on-the-left/

[ryelle]

Are we going to assume that some CSS styles will not work or conflict?

I'm pretty sure that's a gutenberg bug with custom link colors, but haven't tracked it down yet. It's an issue with the current (Twenty Nineteen) display too, so it's unrelated. It was reported here: #235

[kjellr]

Thank you for this change! It looks great.