windows defaulted to dark theme - there could be several reasons for this

if it took a very long time then it might well of been the update from 1903 to 2004 which is a 'biggie'

the age of the laptop in terms of the latest available drivers, the amount of memory, the speed of the processor, the speed of the SSD/HDD and physical condition of all these may slow things down, as might things running in the background security wise etc

if windows finds it has to make do with less 'space/resource' than it really needs to organise the update it will 'let go' of certain settings and reset - this can also happen if some drivers are not as uptodate as windows would require in regard to the features they provide, even if these drivers are the latest available

this may be what has happened here, it is a lot more common than you might think

it is only a personilisation setting, irritating yes, but nothing to be concerned about