There is no easy fix for this. If you convert the database to an .accdb, you can switch to themes but even that will be a pain. You can do it a form at a time or you can write code to modify the various form properties.
Themes are great for creating new forms but you can't impose them on existing forms - Maybe Luke Chung of FMSINC will see this and create a new tool for us. When you have an old form in a new database, Access assumes that the control properties were customized by you and that's the way you want them. To make the form follow a theme, you need to change all the font and color properties to "theme" values. So your code would need to change #FFFFFF to Text 1, Lighter 80% for example. I would probably not attempt to customize the conversion and just go with something more generic. Like pick a font and apply it across the board. Then pick a font size. Then pick a color for labels, data, and backgrounds. You'll need to get rid of background images also if you used them That will sort of standardize them and you can tweak them one at a time.