  We characterise the properties and evolution of Bright Central Galaxies
(BCGs) and the surrounding intracluster light (ICL) in galaxy clusters
identified in overlapping regions of the Dark Energy Survey and Atacama
Cosmology Telescope Survey (DES-ACT), covering the redshift range
$0.20<z<0.80$. Using this sample, we measure no change in the ICL's stellar
content (between 50-300\,kpc) over this redshift range in clusters with
log$_{10}(M_{\rm 200m,SZ}$/M$_{\odot})>$14.4. We also measure the stellar mass
- halo mass (SMHM) relation for the BCG+ICL system and find that the slope,
$\beta$, which characterises the dependence of $M_{\rm 200m,SZ}$ on the BCG+ICL
stellar mass, increases with radius. The outskirts are more strongly correlated
with the halo than the core, which supports that the BCG+ICL system follows a
two-phase growth, where recent growth ($z<2$) occurs beyond the BCG's core.
Additionally, we compare our observed SMHM relation results to the IllustrisTNG
300-1 cosmological hydrodynamic simulations and find moderate qualitative
agreement in the amount of diffuse light. However, the SMHM relation's slope is
steeper in TNG300-1 and the intrinsic scatter is lower, likely from the absence
of projection effects in TNG300-1. Additionally, we find that the ICL exhibits
a colour gradient such that the outskirts are bluer than the core. Moreover,
for the lower halo mass clusters (log$_{10}(M_{\rm 200m,SZ}$/M$_{\odot})<$14.59
), we detect a modest change in the colour gradient's slope with lookback time,
which combined with the absence of stellar mass growth may suggest that lower
mass clusters have been involved in growth via tidal stripping more recently
than their higher mass counterparts.
