Based on the answer in this link, Inno Setup how to change the font color or style of label on TInputFileWizardPage
This is the code to change the font style,
PageFileSelect.PromptLabels[0].Font.Color := clRed;
PageFileSelect.PromptLabels[0].Font.Style := [fsBold, fsItalic];
But how to change the font name of the entire installer dialogs and controls in one go?
Both comments above are correct.
It is quite complicated to to this in vanilla Inno, but there are several 3rd party plugins e.g. VCL Styles or Graphical Installer (check details here) which provide enriched API for this purpose (to make everything easier).
There is a lot of official examples what to do, but if you have any specific question feel free to ask here on SO (I am Inno contributor and developer of Graphical Installer).
The answer is provided in this link:
Change the entire font name of all dialogs (part 2)
Use DefaultDialogFontName directive:
[Setup]
DefaultDialogFontName=<your font name here e.g. Showcard Gothic>