If you are willing, you could consider this:
Create (or edit)
userContent.css using a simple text editor (since it should be a simple text file) in the following path:
****** is the profile unique to each user.
You may also have to create the
chrome folder since it most likely will not exist in more recent versions of Firefox.
In that file place the following:
-moz-appearance: none !important;
color: #000000 !important;
All it does is to force a black font color in input boxes on the google.com site. If you want that code to apply globally, remove the first line and the outer curly brackets.
Please note that
userContent.css are case-sensitive.
Alternatively, you could install the Stylish extension and then browse the gallery of styles available. There is a well attended forum available as well.
The advantage (or disadvantage) of modifying the userContent.css (and userChrome.css) files or using the Stylish extension is that the settings in your stylesheets or in Stylish will have the last word and not the theme. So you can change themes without affecting the way your Firefox looks.