Theme Search: Add an icon or button to indicate the trigger of the search

[epeicher]

After the deploy of https://github.com/Automattic/wp-calypso/pull/68445, the search for themes is triggered by pressing Enter and that could be confusing to users that are used to search as you type functionality.

An icon, button or any other visual clue should be added to the Search input when searching on enter to indicate that that the user should press the button or press Enter to trigger the Search.

This link is a suggestion of potential functionality https://nikitahl.com/search-icon-inside-input

Related context can be found here

• https://github.com/Automattic/wp-calypso/pull/68445#issuecomment-1263811822

This will be similar in functionality to the following task for Plugins Search:

• https://github.com/Automattic/wp-calypso/issues/63451

Related to #68445

[cpapazoglou]

@vinimotaa , @SaxonF we have the same experience in both /themes and /plugins where a search button is missing. Can you propose a quick solution here?

[cpapazoglou]

Related P2 pdh6GB-2Is-p2

[epeicher]

The related P2 changes pdh6GB-2Is-p2 will be implemented as part of the Marketplace Search:

• https://github.com/Automattic/wp-calypso/issues/63451

After that PR is completed, this task could be progressed by Marvel team