OK, think I worked out what's happening on your side.
There are multiple ways to add comments to a PDF.
1. Sticky note comment.
You click on the sticky note icon (looks like a speech bubble), click somewhere on the page, add your text and then click the Post button. In dark theme, your text will be a light grey. In light theme, your text is a dark grey.
2. Adding text to the page through the Text box (T inside a box icon).
You click on the text box icon, draw a box on the page, and type in text. With this text, you have the option to change the text colour. If you change the colour, the text on the page will show that colour as well as in the comments area on the right. So, you could potentially have blue text on a dark grey background.
Your solution could be to change to a light theme (View > Display Theme > Light Gray) or change the colour of the text in the text box comments.