Prismatic adds its own settings page at Appearance → Prismatic Settings. It has three tabs.

General

Reading

• 404 Page: choose a page to show when someone visits a link that doesn’t exist. Set that page to Private so it doesn’t appear anywhere else on your site.

Clean ClassicPress

These options remove things many sites don’t need, which makes your pages a little lighter:

• Disable Emoji Scripts: all modern browsers support emoji, so this removes the extra JavaScript loaded on every page.
• Disable Toolbar: hides the toolbar on the front end of your site, along with the JavaScript and CSS it loads.
• Disable ClassicPress Embeds: turns off the embed scripts.

Click Update Settings to save.

Themes

Shows information about Prismatic.

Plugins

Lists plugins that work well with Prismatic:

• Backdrop Custom Portfolio, which powers the Portfolio section of the Showcase template.
• ClassicPress Directory Integration
• Regenerate Thumbnails, which is handy after switching themes so your existing images are resized for Prismatic.