Formatting Word output in DocToHelp is done be editing the appropriate target template. Target templates are different from source templates, since the author needs different tools for source documents and manual documents.
Target templates can be modified, or even exchanged by own templates.
In source documents styles may have a different appearance*, especially since Word has a lot of options to influence appearance of text.
* I assume that this is what you name "design mode", or do you edit xml files in an older D2H version?