I would recommend not to work with the river theme at all, since it´s deprecated:
Plus hardcoded changes to the themes are not recommended for compatibility reasons.
Why is it deprecated?
In this case how can I create a new theme from sratch?
Is there a reason why you don't / can't just change the style of the footer in that theme and then save it off as a new theme? See here for more information:
Doing what you are doing, assuming I am understanding it correctly, is not a supported method of using FileMaker Pro and potentially could cause problems down the line.
You best approach is not to try and alter the standard theme, but save your modified version as a new custom theme.
I don't think he is talking about hacking the theme itself. instead used the base image to create a new one.
It just needs to be inserted into the footer as an image and splice it.
How you work with themes depends on what you want to do. For instance, do you plan to use this amended theme for just one database, or for many different databases? Are you going to have multi-file databases that need to have an overall motif?
When you save a custom Theme in FM, it gets saved in the DB file you're working with, not in an external folder. To use it in a different DB, you have to import the theme (File/Manage/Themes) from the first database. You don't want to start a theme from scratch (don't reinvent the wheel), but you can change a theme around to your heart's content. So by all means save your modified version as a new custom theme.
I assume you've tried saving themes and styles in the Inspector. One thing I found was that, after setting up a new theme in one layout, when you go to apply that theme to other layouts you lose some of the design elements that you've put into those other layouts. To avoid losing them, immediately Undo, and the alterations in your design elements will reappear, but the theme will now be your new custom theme; you can then save those elements as customs styles or changes to the current style of the custom theme.
I think the main reason the old themes should not be used is that they aren't compatible with WebDirect. Use the new themes and your web direct layouts work almost as nicely in a browser as they do in Pro or Go.
Also the new ones are work better together as it seems I can switch from one to another and the parts and sizes remain as they were, just the colors, etc. change, something that didn't seem to work well with the old themes.
Since my initial attempts many, many years ago to add color to my layouts brought the response, "Lots of Colors!" and seeing how a youngster just graduated from the local art academny did so much nicer, I find comfort in the new themes.
One important note, opening a 13 file with 12 can be a problem...