  Continuum reverberation mapping probes the sizescale of the optical
continuum-emitting region in active galactic nuclei (AGN). Through 3 years of
multiwavelength photometric monitoring in the optical with robotic
observatories, we perform continuum reverberation mapping on Mrk~876. All
wavebands show large amplitude variability and are well correlated. Slow
variations in the light curves broaden the cross-correlation function (CCF)
significantly, requiring detrending in order to robustly recover interband
lags. We measure consistent interband lags using three techniques (CCF,
JAVELIN, PyROA), with a lag of around 13~days from $u$ to $z$. These lags are
longer than the expected radius of 12~days for the self-gravitating radius of
the disk. The lags increase with wavelength roughly following $\lambda^{4/3}$,
as would be expected from thin disk theory, but the lag normalization is
approximately a factor of 3 longer than expected, as has also been observed in
other AGN. The lag in the $i$ band shows an excess which we attribute to
variable H$\alpha$ broad-line emission. A flux-flux analysis shows a variable
spectrum that follows $f_\nu \propto \lambda^{-1/3}$ as expected for a disk,
and an excess in the $i$ band that also points to strong variable H$\alpha$
emission in that band.
