It is not possible to edit the CSS of any theme directly really (nor recommended, unforeseen consequences later when you update filemaker).
You should make your changes to an object, save the style to that object, and then save that object's style to the custom theme. This will cause the style for any object (EG tab control) across your file where that theme is used to update.
Essentially it's the same thing as editing CSS, make the changes in one place, and have it apply to all objects across the file.
Thanks for your response but this is not my problem. I do know how to work with styles and themes in the inspector. My problem is that some styles are not editable in the FileMaker inspector but only in the CSS file. As a custom theme does not have a CSS file to edit, it's impossible to change such a style. I was wondering if there is a way to still access this information and edit it. From the binary data of a .fmp12 file I can see the custom theme is embedded but of course I cannot edit it there.
There is no CSS file you can edit for custom themes as custom themes are embedded in the FileMaker file. It is possible to edit the CSS files of default themes, and even to make copies of the default themes, but you'd need to copy the edited CSS and Manifest files to any users, so it's not really practical.
In layout mode if you right click on the Tab Panel there is an option in the Tabs Control Setup dialog for Tabs Share Single Style. If this is de-selected you can apply different styling to each Tab Panel in the Tab Control separately.
I hope this helps