Themes admin page: make theme details, active, and preview links always visible

[joedolson]

Follow up from #52649

In ticket 52649, we fixed a number of accessibility issues in the theme navigation. In the course of discussing that, there was a proposal to modify the layout so that the three action buttons for a theme were always visible, without obscuring the theme screenshot.

Since it was beyond the scope of the original ticket, we opted to complete that ticket and open the design issue as a new ticket.

See:
https://core.trac.wordpress.org/ticket/52649#comment:15
https://core.trac.wordpress.org/ticket/52649#comment:27

[oglekler]

@joedolson without visual representation, it is difficult to decipher what was the scope of the previous ticket and what is left for this one. Can you please clarify the task at hands? Thank you!

[Travel_girl]

@joedolson I'm on this task and work on some mockups, regarding on the disscusion from #52649

[oglekler]

There is no patch and no time before Beta 1 to finish and test it, so, I am moving this ticket into the 6.5 milestone. I hope everyone is fine with it.