It is recommended to install the extension Start TYPO3 Responsive +Customer (startcustomer).
It extends Start with an user-interface for managing a lot of CSS properties by mouse clicks.
It delivers five templates - five themes.
If you want to adapt Start to your needs, startcustomer seem's to be the best base: copy it for example to startmycustomer an adapt it to your needs.
See: Start TYPO3 Responsive +Customer (startcustomer) https://extensions.typo3.org/extension/startcustomer/