This is the expected behaviour. You are editing the 'string' named style which is used for all filetypes. Instead you should just override a style in filetypes.php.
But this is a string, not white space! Why all the white space becomes highligted as string?
Besides, there is a separate definition of white space
white_space=0xc0c0c0;0xffffff;true;false
but it is not working.
I thought it would be convenient to change the setting one time in filetypes.common for all file types I code: PHP, python, html, javascript, css.
And if I change the setting for string in filetypes.php, why not highlight all the white space inside php script in pink, according to Geany logic?