In most cases, there are no options at all to change the content presentation.
If there are options to change content presentation, those options are generally too limited in effect to meet the need.
Setting background and text color, for instance, but not having options to change text size or spacing.
If there are options to change content presentation, those options are often difficult to set by users with disabilities.
Drop-downs - which are generally used to for selecting colors - are cumbersome for most users and present particular problems for the visually impaired and motor impaired.
Users are burdened with designing the presentation by having to manually pick colors for different aspects of content
Often those aspects of content are not defined well and the end effect is unclear
Often there is no preview of the effect the user has picked, resulting in a painful process of having to apply the results to see the total effect, only to discover that is not what the user wanted, and then having to repeat that process with many clicks and roundtrips to get it right.
If the user decides they do not like their preferences and wants to remove them or start over, that sometimes means a laborious process of manually changing every preference setting back to default.