Modify CSS Popup

Hi,

First of all, thank you for the great plugin. It makes sharing through mail very easy. I was looking a way on how to modify the css template for the popup. Because the default background is dark grey while the fonts are black and quite small. I have difficulty reading it myself since I used glasses.

Please kindly to give some guide on how to achieve it, and on which file changes can be made so the popup template can be easier to read.

Thank you in advance.

https://wordpress.org/plugins/wp-email/