I'd suggest rolling (the sensor itself) with layers of a dark tape, (such as brown not black), until you get the sensitivity down. A resistor will not change the sensitivity. A single resistor will also not change the analog to digital voltage reading value, because there is minimal current flowing through the sensor output line.
You could try a voltage divider scheme, using two resistors (similar to the idea shown on the Adafruit link), but that defeats the purpose of the TSL257 as a single package solution for screen triggering, no external components required.
One such suitable tape might be the Scotch Type 35 PVC tape in various dark colors such as brown or blue.
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