@Bader brought up a good new and much needed feature in my opinion. I just quote the message from Discord so its here and can get tracked and does not get lost 
I wanted to bring to your attention a significant issue regarding accessibility. Currently, Logseq does not offer a way through the user interface to change the font, font size, or line-height. This limitation poses a significant inconvenience for users with impaired vision, who may need to adjust the display settings to meet their needs.
Introducing a simple method to modify these settings would be one of the most effective ways to improve accessibility within the application. It is important to consider that it is unreasonable to expect users with vision impairments to invest time and effort in learning the intricacies of Logseq’s CSS or rely on external plugins to make these necessary adjustments.
To create a more inclusive and user-friendly user experience, please consider implementing this feature in a future update. Your commitment to enhancing accessibility will undoubtedly benefit not only those with vision issues but the entire Logseq user community.
Thank you for your attention to this matter, and I look forward to seeing these improvements in the near future.
Example case:
Using themes and custom css together?