Themes decide the appearance of a website. Select your favorite themes at “Admin Panel -> Appearance -> Themes”.
Click “Activate” to use a theme. Click “Customize” to adjust in detail.
Each themes use different thumbnail size, but the system won’t regenerate new size of thumbnail after changing the theme. So you should regenerate thumbnails. Regenerate thumbnail at “Admin Panel -> Tools -> Regen. Thumbnails” after changing the theme each time.