That a movie-file is .avi doesn't tell what codec was used to create it.
Anyhow (if it concerns
external textbased subs
(such as e.g. a .srt-file)):
default settings (such as for font/colour/background) for subtitles (primary and secondary) can be set under:
(rightclick >) Options > Preferences > Subtitles [Subtitle properties]
settings for an individual movie (when the movie and subs are loaded):
(rightclick >) Subtitles > Primary/Secondary > Subtitle properties ... etc.
Of course it it always possible that the subtitles are "
burned into the movie", in which case you cannot change or remove them (as they then are simply part of the movie-picture).
If the "burned-in" subs are not so clear, you could perhaps "superimpose" external subtitles on them with a non-transparent background (uncheck/deselect "Transparent background" under [Subtitle properties]).