Hi Viking. When using extract, I suggest using the lightest extract available and adding flavors, roastiness, and coloring through specialty grains. First, we don't know what is in the dark malt (aside from the base malt) and since you don't know, it makes it very hard to decide what specialty grains to use and how much.
Secondly, its analogous to all-grain brewing. Extract = your base malt. In other words, all-grain brewers start with a base malt and then add things to it. And they often start with the same base malt for a whole variety of styles. A brewer might use Maris Otter base malt for a pale ale, an IPA, a bitter, an ESB, a porter and a stout.
The extract is like the base malt - its the foundation upon which you build the style, by adding other grains and ingredients to it.
As to partial mashing or steeping, partial mashing would be fine, but I don't think you need it in this recipe, I think steeping would work. It doesn't look to me like you're needing to get fermentables out of your specialty grains, just flavors and colors.