Yahoo have designed a very poor User Interface when it comes to the new look of Mail. First of all, the typeface is far too light as there is not enough weight on the font to differentiation between the background and the text!
Secondly, Users like myself are finding it impossible to differentiate between the 'unread' mail and the opened mail as the typeface Yahoo have chosen is unsuitable to see what is bold and what is not bold. They have tried to assist by adding a 'blue dot' to indicate an email is unopened, but that is not what the User expects to see, nor how we work!
And the arrogance of support staff asking the User to 'increase the browser magnification', is reproachable. Listen to us Yahoo!