You can also alter web page appearance with user CSS (with this solution, however, you will not be able to change appearance with the click of a button like the add-on probably does).
Create a file called userContent.css in the chrome subfolder of your Iceweasel profile folder (create a folder called "chrome" if it doesn't already exist). Type your CSS in that file, save the file, and restart Iceweasel.
I use the following (which sets pages dark); you can use it as a template and just change the colors if you don't know CSS:
body, div, p, ol, ul, form, section, span {
color: #808080 !important;
background-color: #050B11 !important;