Selecting best theme which is compatible with all extensions and easily customizable is good initiative while starting any ecommerce project. I have found the list of best Magento themes here, All the listed themes are the best selling in 2018 and easily customizable through CMS or html CSS.
Porto | Ultimate Responsive Magento Theme
Ultimo – Fluid Responsive Magento Theme
Fastest – Magento Multipurpose Responsive Theme
Market – Premium Responsive Magento Theme
Novetty – Fastest Magento 2 Theme
YourStore – Premium Magento 2 Theme
Athlete – Fluid Responsive Magento Theme
Hope, the list will help ecommerce store owners or startups to select any of them.