Update Modal: make the CSS consistent with the release blog

### Impacted plugin

Jetpack

### What

Make the CSS consistent with the release blog

### How

The update modal CSS does not match the release blog theme's CSS. For example, the theme displays the button text in all caps while the update modal does not.

The CSS used by the release blog and update modal should match. This will allow the content author to easily observe how their content will look in the update modal.

Start by locating the update modal CSS and the release blog theme CSS in the Jetpack plugin, then compare their styling, including button text capitalization. The work is done when the modal CSS matches the release blog theme closely enough for authors to preview the same appearance.

Written by the indexing model from the issue text.