For #1, if you're saying you want to change the design of your theme by using CSS you can do that in the Shopify admin at Online store > Themes > Actions > Edit code.
If you're not familiar how to edit themes or find what CSS to edit, I have a guide: How to Safely Edit Your Shopify Theme.
For speed optimization, the usual suspects that slow down a site are:
- theme not optimized
- lots of 3rd party apps
To get started on optimization, run your site through the Shopify Analyzer. That's a tool my team built specifically for analyzing and optimizing Shopify sites. It will give recommendations and tips how to optimize.