Please follow below steps
1. Create a new theme
2. Got to theme properties
3. Goto Relationship tab
4. In the Style Inheritance box deselect the 3 inherit options
5. In the relationship Text font click on A on the font options
6. In the Relationship Label font editor change the size/color as per your requirement and apply the theme on the model level or diagram level or Subject Area level as per your requerement,