No, you cannot set default themes for individual forms. The entire point of themes is that they apply to all forms. However, you may have been too heavy-handed in modifying the theme code.
First question: Did you substitute the form title Tax Invoice or are you using a built-in tax code that makes that substitution for you? (Some built-in tax codes, where it is known that regulations require a sales invoice to be titled tax invoice, make that substitution without user intervention.)
Second question: If you substituted the title, did you do a brute force approach (which is what it sounds like) or did you use a replace filter? The latter technique will not change titles of other forms unless they contain the work Invoice. See this Guide:
https://forum.manager.io/t/changing-a-form-title/7877
Third question: If your regulations require use of the title Tax Invoice, have you tried just using your country’s built-in tax code? Does it change the form title on the default theme? If so, you don’t need the custom theme just to change the title. If not, but local law actually requires the title Tax Invoice, let us know so the program can be updated. Furnish a legal citation from your tax authority.