Appearance Themes: Use text to open theme details instead of an icon

[nacin]

The "i" icon when opening theme details is a bit vague and probably not the best use of the information icon. There was some concern it could be lost in translation, especially since it is being used as a primary action, versus a simple visual next to a notice (for example).

After a long conversation in IRC involving many (​log), as well as feedback from some translators (​link), we're going to go with text here, instead of an icon. Something like "theme details".

[shaunandrews]

Removed the icon and added a text label that appears on hover.

[nacin]

In 26708:

Themes: Switch to a text overlay instead of an icon on hover. Numerous reasons outlined in the ticket.

props shaunandrews.
fixes #26440.