[ticket/11329] Moving colors to colours.css

Moving color values from buttons.css to colours.css

PHPBB3-11329
Merge remote-tracking branch 'remotes/cyberalien/ticket/11329' into develop

# By Vjacheslav Trushkin
# Via Vjacheslav Trushkin
* remotes/cyberalien/ticket/11329:
  [ticket/11329] Moving colors to colours.css