The screenshot shows a minihtml popup. Fonts are defined via stylesheets, which are defined by the plugin which creates the popup.
Theoretically a color scheme can override those styles in order to create a consistent look and feel for all popups of all plugins. The mechanism isn’t very reliable or convenient in practise as it means compiling stylesheets to JSON (sublime-color-scheme) and seems to have a couple of edge cases, which tends to break more than it helps. It’s therefore not commonly used.
In case the popup is created py
mdpopups library you might be lucky to change the font settings via
Otherwise you’d have to investigate which plugin creates the popup and whether it provides customization support.