@Ovismue-Veholkin ,
I have a personal User Defined Language (UDL) for TI-Basic / Extended Basic for the TI-99/4A. It doesn’t currently use those extensions (I’m not overly connected to the modern TI-users world, so I didn’t know there was a standard extension for TI-Basic files), but it highlights the syntax. I haven’t uploaded it to the public UDL Collection because I didn’t figure it would be of general use, since I don’t know how many Notepad++ users intersect with 99ers. So you can download that UDL definition into your %AppData%\Notepad++\userDefineLangs\ folder, then restart Notepad++ to make it see the new UDL) – and then you can manually add 8xp 8xk to the Language > User Defined Language > Define Your Language dialog’s User Language: TI-BASIC selection, in the Ext.: input which currently has TIBAS (so you would change it to TIBAS 8xp 8xk or just 8xp 8xk). (That said, I also am just assuming that by “TI-Basic”, you mean the BASIC that was used on the TI-99/4A; I don’t know if that name has a different meaning to other people. And if those extensions are for binary files that encode/store TI-Basic, like maybe for emulated “tape” storage or something, it won’t help, because Notepad++, as a text editor, has no way of understanding a binary file.)
As an aside: When we created the Themes Collection, I uploaded a theme that you might find amusing (though given that it’s inspired by the default TI-99/4A bright-cyan background color, I doubt anyone would want to use it on a regular basis. And no, when I made my UDL, I used standard light-theme color scheme, rather than a TI-inspired color scheme)
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addenda: after making this post, I did a websearch, and found that 8xp appears to be the file extension for TI-83/84 calculator’s BASIC – confusing of Texas Instruments to use the same name for what are likely two totally different implementations of BASIC. My UDL probably won’t help you; sorry. And it wouldn’t surprise me if the 8xp/8xk extensions were binary files that compress the BASIC text into bytecodes/opcodes that the calculator uses internally; in which case, you would need an external converter to convert from binary to text, then edit the text in Notepad++, save it, then use a converter to convert it back to the binary format.