1) Go to Dashboard > Plugins > Add New. Search for "simple custom CSS".
2) The first result should be a plugin named "Simple Custom CSS". Click the "Install Now" button.
3) The plugin will download and install itself, and on the resulting screen, click on "Activate Plugin".
4) When the plugin is activated, you'll see a new option in Dashboard > Appearance, labeled "Custom CSS". Selecting that option will take you to the plugin's interface.
5) You'll see a text box with the text "/* Enter Your Custom CSS Here */". Delete that text and put this instead:
#site-footer { height: 48px }
6) Click on the "Update Custom CSS" button. If you get the message "Custom CSS updated successfully", it worked. Go back to your site and your problem should be fixed.