Missing fields validation messages not showing

v. 3.7.2

The positioning of the missing fields validation messages are appearing only as plain white boxes aligned to the left of the browser window on many of my themes,

I have tried for hours unsuccessfully to change the styles.css of my theme and the styles.css of the plugin. I can get the box positioned under the filed entries but no text is visible which should say ” Please fill the required field.”

please could you advise

https://ww.wp.xz.cn/plugins/contact-form-7/