What is this?
让 TextMesh Pro 富文本 Mark 标签生成的颜色置于文本后方，而不是覆盖在文本之上。
Place the color generated by the TextMesh Pro rich text Mark tag behind the text, rather than overlaying it on top of the text.
如果你想修改 Mark 标签生成的背景色的上下左右偏移，请使用 padding 属性
If you want to adjust the top, bottom, left, and right offsets of the background color generated by the Mark tag, please use the padding property.
Reference
https://forum.unity.com/threads/how-to-set-text-background-color.570751/#post-9538573