Reg. White color text in textbox

Your product designer is remarkable, however, it's just the start and I am stuck up somewhere.

I added a product and on designer page when I enter quantity it's white coloured and nothing is seen. When I highlight the quantity entered then only text can be seen. May be due to the theme I am using.

Please tell me what code should I use in customer CSS as !important that the CSS overrides the theme and text in quantity text field is shown perfectly in black colour.