To do that, you’ll have to know a bit of CSS.

For that kind of pop-up (the built-in error message box), you’ll have to create some CSS that targets the pop-up element class (You can find this by right-clicking and hitting inspect, then moving through the HTML tree and finding your pop-up error message element when the message appears) for the application, since you don’t have access to the pop-up’s ‘style’ property.

There are a few ways to add CSS, but a straightforward way is to go to (in Documents) [project] > theme > web, then create a new SCSS file with your styling, and then import that new file in the ‘main.scss’ file there.