Firefox:

If web forms and controls appear dark and unreadable you need to edit the css files that Firefox uses. The file location may vary on different linux distributions.

1. Backup the original file:
sudo mv /usr/lib/firefox/res/forms.css /usr/lib/firefox/res/forms_backup.css

2. Replace it with the provided forms.css
sudo mv /<downloaded location>/Neutronium-GTK2/forms.css /usr/lib/firefox/res/forms.css

3. Then you need to edit html.css:
sudo gedit /usr/lib/firefox/res/html.css

Around line 60, change:

body {
  display: block;
  margin: 8px;
}

To:

body {
  background-color: white;
  color: black;
  display: block;
  margin: 8px;
}

Save the file, then restart Firefox, and you should have standard colours.