MINOR: stats: Add dark mode support for socket rows

MINOR: stats: Add dark mode support for socket rows

In commit e9ed63e548 dark mode support was added to the stats page. The
initial commit does not include  dark mode color overwrites for the
.socket CSS class. This commit colors socket rows the same way as
backends that acre active but do not have a health check defined.

This fixes an issue where reading information from socket lines became
really hard in dark mode due to suboptimal coloring of the cell
background and the font in it.

(cherry picked from commit a690b73fba4d09ff32bc9c1e19c321ae0649a7cd)
Signed-off-by: Willy Tarreau <w@1wt.eu>