[ticket/12563] Changes two style language variable for better understanding

Little changes on two language variables. 

PHPBB3-12563 -  https://tracker.phpbb.com/browse/PHPBB3-12563
@nickvergessen Weren't we changing things like this to be less verbose, like: 
`<a href="%s">Return to installed styles list</a>.`
`<a href="%s">Install more styles</a>.`