button hover style fix
added configs for style custom

'GSADMINTHEMEFILE'      => 'admin.xml',                   // (str)
custom admin xml theme file name
'GSADMINTHEMEENABLE'    => true,                          // (bool)
custom admin xml enabled
'GSCSSCUSTOMFILE'       => 'admin.css',                   // (str)
custom css file name
'GSCSSCUSTOMENABLE'     => true,                          // (bool)
custom css enabled