Note that this refers to an older Joomla version. The problem is caused by a conflict between CB template and site template. The site template overrides the CB template. In Joomla 1.x, look for layout.css in your active template folder.
If you're not sure which element you are using, try finding it with Firefox and Web Developer extension - Information -> Display Element Information. Click on the text field you want to change and it will display the CSS for it. Copy/paste the element info into your layout.css file and insert the parameters you want to change.
In some templates, the css may be located in template.css or other CSS file, but it will be somewhere in your active template folder. In J2.x-3.x, most templates will have several CSS files. FF & Web Developer should help you identify the code for the element, and a quick search should get you to the proper file to edit.
