Onetastic 5.5.0 update allows editing your custom styles to fine tune them. It also comes with new dark mode support for OneCalendar.
Once you click on the Edit button, the controls in the dialog will be enabled and you can change the style as you want, like changing the font to Arial and color to red:
As before, you can use the checkboxes to choose which properties should be part of your style. Properties that aren't part of a style won't be affected when the style is applied to a piece of text.
Once you are happy with your changes, use the Save button to save it. You can then select another style to modify. If you want to revert your changes, use the Cancel button.
Here is how OneCalendar looks like in dark mode: