[MA-13]: Updated the Theme list section in the settings modal

[ayush-chauhan233]

Summary

• Updated the premade theme and custom theme radio button accordion.
• Replaced divs with buttons to ensure keyboard navigation for the premade themes.
• Updated the accordion in custom theme section and added aria-expanded and aria-controls attributes.

Steps to reproduce

• Navigate to the the Display tab control using down arrow key.
• Now navigate to the Theme edit button and select it.
• Now navigate to the Custom Theme radio button using arrow keys.
• Now navigate to the Sidebar Styles button and notice that the control is not reachable by the keyboard.

Expected Behavior

• The premade theme and custom theme radio button accordion must be keyboard accessible as well its content.

Ticket Link

Jira https://mattermost.atlassian.net/browse/MM-61592

Screenshots

Before	After

Release Note

NONE