In language/en/acp/styles.php are the language variables:

'STYLE_INSTALLED_RETURN_STYLES'
'STYLE_INSTALLED_RETURN_UNINSTALLED'

Both are displayed in the "success"-message after you have installed a new style (see attachment).

So i think the names are wrong, especially the second one.

Maybe something linke:
RETURN_STYLES_LIST
and
RETURN_UNINSTALLED_STYLES_LIST
would be better.