Hi,
themes are generally applied to a presentation its entirety. As far as I
know, there is no way to apply a theme to a selection of slides only.
What you could do, however, is create two different presentations, one
with the first few slides in theme 1, the other one with the subsequent
slides in theme 2. In either one of these presentations, click on
Insert>Slides From>Other Presentation. Select the other presentation in
the Finder dialogue that opens and make sure the option "Select slides
to insert" is selected. In the next window, select the slides you want
to insert (hold down the Shift key to select multiple slides) and make
sure "Keep design of original slides" is checked. That should do the
job. You can now switch to Slide Sorter View (View>Slide Sorter) to
rearrange the slides and place them where you want them to appear.