Hi Dani,
You are always welcome here 
1. I was thinking that Dazzling had the little Custom CSS area in Theme Options. Would I also need that plug in?The plugin is an alternative if for some reason the Custom CSS option of the theme doesn't work for you so if the theme option works then you don't need to use the plugin.
2. I have a question about getting the slider across the top to stay (or appear / be constant) when someone clicks on a post. Can you help me with that or prefer I start a different thread (I’m thinking I did at one time…)
You can try using the solution posted in the following reply to achieve it.
Best Regards,
Movin