Using MarsEdit's template editor, you can customize the look of the preview to match the look of your published blog. A template can be automatically generated to get get you started:
Once you have a baseline template for your blog, we recommend hand-tuning the HTML to remove superfluous content such as any ads, comment fields, etc., that may be cluttering the preview window's appearance.
While editing, use the Insert Placeholder pulldown menu at the bottom left of the template editor to insert different elements into your template. These will be replaced with the actual values from your blog posts when previewing in MarsEdit.