I was as happy as just about everyone else when a distraction-free Reader Mode was added to Chrome OS 75. But the implementation was, and still is, pretty basic. A feature request to “adjust the font family, font size, and page theme” however, will help modernize Reader Mode and this work in progress is slated for Chrome OS 79.
The above image is an example of the current Reader Mode implementation; it strips out ads and scripts to let you focus solely on the post content. But you can’t modify the font or font size, for example.
In contrast, take a look at this nice menu option provided by the Read Now with Instapaper extenstion used on the HP Chromebook X360 14 sale post from yesterday.

I can adjust the page background color, font family, font size and border spacing, which is really what the native Chrome OS Reader Mode should offer.
This feature request can be tracked and starred if you want to follow along. I’m doing that because it’s not yet clear exactly what options it will bring to Reader Mode just yet. At this point, it appears font size and border spacing are in the plans, however. If you have specific requests for this feature improvement, now’s the time to hit the bug report and let the developers know!