Slide master

Greetings, I am trying to build a slide master after viewing a tutorial hosted by Dave. I imported two slides from the content library into my storyline line project. I switched to the slide master view and the slides appear. My questions is how can I change the color themes from the import slides to match my color theme and fonts? Also I imported two slides yet the entire template appeared when I only need two. I am deleting the unwanted slides one by one as selecting multiple slides at once to delete does not work. Can I delete multiple unwanted slides at once?