Inconsistent styling of "place block" button

Problem/Motivation

When the "Place block" mode is activated, this module adds a small button with a plus sign in each region that allows users to add a new block. The styling of this button inherits from the theme, which can result in inconsistent appearance and unwanted style conflicts, depending on how the theme has styled the .button classes.

Steps to reproduce

- Enable Place Blocks module on a site
- Apply Olivero or core theme
- Click "Place block" in upper right
- Observe appearance of button
- Apply various contributed or custom themes and click "Place block" again
- Observe button looks slightly different or conflicts with theme's button styles depending on the theme

Proposed resolution

Create a custom class with the appropriate styles for the add block button and remove the generic .button classes from markup so that it doesn't depend on a theme's .button styling.

Remaining tasks

Update CSS
Update markup

User interface changes

Match basic styling in Olivero. Users should see a consistent, simple plus button for adding new blocks.

API changes

none

Data model changes

none