Understanding how to modify formats
Make changes in appearance, such as font, style, or alignment, in local or document-specific format configuration files (never in the master format configuration files, nor in your project configuration file).
For example, in %omsyshome%\d2g\system\formats\d2htm_formats.ini you will find the following definition of the format for top-level numbered list items:
[Numbered1] based = Body margin top = 4pt margin bottom = 0pt margin left = 2pt list style = decimal list level = 1 number = List1Num xref = NumXref
If you want more space above each list item in HTML output, you would add the following setting to %omsyshome%\d2g\local\formats\local_d2htm_formats.ini:
[Numbered1] margin top = 12pt
This setting would override the space above for all HTML outputs from all DITA documents. (You must include this setting in a format configuration file; it will not take effect in your project configuration file.)
Formats can inherit properties from other formats, so this change would automatically carry through to the rest of the numbered formats, because they are based on the definition of Numbered1:
[Numbered1First] based = Numbered1
[Numbered2] based = Numbered1
However, this change would not affect any added paragraphs under each numbered item, because their definition overrides the space-above property:
[Unnumbered1] based = Numbered1 margin top = 2pt
Format definitions form a tree, with inheritance, so you can adjust a particular branch, in one place. The format configuration files themselves also form trees, where the scope of a change is determined by which file you modify. See §39.4 Deciding which configuration file to edit.