Currently, CometChat only has one theme. You can change the Theme Color, height, Width of CometChat. Please refer below documentation URL for your reference.
# Change Appearance/Color For CometChat
# Position Of CometChat Docked Layout
Please follow below steps to Change Docked Layout Height & Width.
Login to CometChat Admin Panel using your credentials.
From the Left side of the menu click on "Settings->General".
Search for "Docked Layout Settings".
Search for "Chatbox Height (Minimum Height can be 350px)" & "Chatbox Width (Minimum Width can be 230px)".
Change value as per your requirement and Click on "Update" button.
Click on "Clear Cache" from the left side menu.
Clear Your Browser Cache.