Actually when we save some theme then Notepad++ write the fontStyle attribute even if it doesn't exist. This should not be like that because for some style expose/changing this attribute has no effect (read this #12508).

STR:

1. Open Notepad++.
2. Open some theme file, like Bespin and scroll to the end.
3. Go to Settings > Style Configurator...
4. Change default theme to this from point 2 (here Bespin) and press Save & Close button.

We can see on the opened Bespin file that the missing attributes fontStyle have been added as fontStyle="0". This was also mentioned here #12635 (comment).

Notepad++ v8.4.7   (32-bit)
Build time : Dec  1 2022 - 02:39:50
Path : D:\_test\NPPClean\notepad++.exe
Command Line : -multiInst
Admin mode : OFF
Local Conf mode : ON
Cloud Config : OFF
OS Name : Windows 11 (64-bit) 
OS Version : 21H2
OS Build : 22000.1219
Current ANSI codepage : 1250
Plugins : 
    mimeTools (2.8)
    NppConverter (4.4)
    NppExport (0.4)