You can control all the Presets via the theme manager configuration. All 6 presets appear at Admin → Themes → Quasar.
You would then adjust the parameters in Theme-specific Settings to meet your requirements. On selection of a preset, the administrator automatically updates the Settings to match that particular preset. This system allows you to quickly and effortlessly customise a preset to your liking.
To change these preset values, you can simply edit the styles.php file for a specific preset style.
The options are as follows, and the areas that they control are indicative in their name:
In the event that you need to customise all the preset arrays, and do not wish to use the theme administrator, then you can customise the preset arrays in the styles.php file. Do note that it is much easier to use the theme administrator, and editing the preset array is only really necessary if you wanted to use multiple custom configurations on your site which is uncommon.