From our review of a number of discussions on how to deal with this issue, it looks like we are going to have to fix our layouts by taking an available theme, renaming it and then "customizing" it to get it as close to our existing "Custom" theme as possible.
If this is the route we should be taking?
Some themes do NOT work with WebDirect. So, I would consider that first. Then see if what you have works well with FMGo.
The next concern is for 'overhead' on themes where each layout has several independent objects styled. Even if you have the same objects on another layout, the styles are then saved with the layout. You can see how a compact (even custom) theme is stored with the theme definitions and only called on the layouts that use it. The objects follow the 'rules' for the theme and there is no need to store separate styles.
Yes, read all the threads on this topic as I may have over-simplified the process. And make the determinations first based on WebDirect and FMGo.
p.s. I have only 250 layouts to fix what was mostly custom to match company colors and designs.