Change font colour in plugin forms

I’m trying to change the colour of the font in the forms on my ‘contact’ and ‘testimonials’ pages of my site. At the moment it’s grey and I’m wanting to change it to black.

I’ve been modifying colours in the css stylesheet with no success and I’m hoping someone might be able to point out what I’m missing or what I need to add.

My site is:

Many thanks in advance,
Denise ??