Solomon has four visual themes. They are purely aesthetic - your conversations, memory, and everything else remain unchanged regardless of which theme you choose.
The four themes
Serenity - a clean teal-blue tone. The default. Calm and clear.
Vitality - a warmer, coral tone. A little warmer, a little more personal.
Balance - a cool blue. Quiet and subdued.
Nocturne - a deep blue. Subtle and moody.
Changing your theme
Go to Settings and browse the theme options to find the one that feels right. Changes apply immediately.