With Onetastic update 5.7.0, you can now move custom styles to a separate ribbon tab. Until now, custom styles were offered in a gallery in the Home ribbon tab, requiring two-clicks to access them and no way to add them to Quick Access Toolbar (QAT) or customize your ribbon with them.
With the ability to move them to a separate ribbon tab, you can now:
Here is an example of custom styles added to Quick Access Toolbar, which allows you to use Alt+1 - Alt+9 shortcuts for them:
You can choose where custom styles show up (or hide them completely) from Tools and Settings > Onetastic Options or click on Display Options button in Custom Styles Dialog.
If you change OneNote theme, you will need to restart OneNote for the changes to take effect in custom styles.