In tonic sol-fa, the difference between sharps and flats, as used in staff notation, is irrelevant, to be honest.

If you are notating a tune in the key of F major, the fourth note of the scale is a Bb, if you decide that you need to jack it up a semitone to F#, then that Bb note becomes an A#.

It don't matter one whit as far as tonic sol-fa is concerned, because you are still using the same d,r,m,f,s,l,t,d to denote the melody.

I am well aware of the theory behind scales, but when using tonic sol-fa, (which after all is primarily a resource for singers who are not classically trained musicians), differentiating between the accidentals is a pedantically unnecessary complication.