How are you increasing the font size?

Add code to the userChrome.css file below the default @namespace line or use Stylish. Adjust the value (50em) to suit your needs.

• http://kb.mozillazine.org/userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#ColorsDialog { width: 50em !important; }

• create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• use a plain text editor like Notepad to create a (new) userChrome.css file in the chrome folder (file name is case sensitive)
• paste the code in the userChrome.css file in the editor window
• make sure that the userChrome.css file starts with the default @namespace line
• make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userChrome.css.
  otherwise Windows may add a hidden .txt file extension and you end up with a not working userChrome.css.txt file

I added #ColorsDialog { width: 50em !important; } to my userChrome.css file and kept increasing it, up to 800em (!), closing & opening FF each time, but the colors dialog box is unchanged.

My basic font size on dialog boxes, toolbars etc. is set with Windows color, advanced appearance settings. (Windows 7)

My mistake, sorry about the confusion.

I sometimes forget that code for these about pages needs to be in userContent.css and not in userChrome.css.

Tested it this time.

Add code to the userContent.css file.

• http://kb.mozillazine.org/userContent.css

@-moz-document url-prefix(about:preferences#content){
 #dialogBox { width: 50em !important; }
}

Works like a charm! Thanks as always for your help, cor-el! Now that I can see the whole thing, I find it doesn't do what I want it to do - namely, to change ONLY the color of text to black without changing the color of links, background, etc. Any way to do this?