You need to know the name of the element that is the id number.
Open your anki note in Obsidian, Hit CTRL/CMD+SHIFT+i to open the cosole, click the Element tab. Roll up and down over the code in the console and look at the highlighting on the anki note to see if you can identify the id number code. Its probably called something like ID Number.
Once you know that, you can try adding a CSS snippet that changes the size and color of the text in the preview mode.
It will be something like:
.markdown-preview-view [element name] {
size: 5px;
color: gray;
}
You’re going to have to do some work to figure out the details and then post back here to get some specific help.