Most printable planners are drawn once in a design app and edited by
hand every year. That is why you find a Tuesday sitting under a
Wednesday in March, and why next year's edition is often this year's
with the numbers nudged along.
These are generated. Then — the part that matters — the finished PDF
is read back and every date on every grid is checked against the
calendar a second time.
The look follows from the same idea. Thin grey rules instead of
blocks of colour, because you are printing this yourself and ink is
not free. Wide margins, because a planner is somewhere to write.