Hello Donald,
Few more on this subect:
Are the css files from a library of css files that you used? I'm assuming you did not create them yourself. (nobody has that much time)
The default app's theme is Bootstrap CSS with very little modifications. The other includes themes are downloaded from Bootswatch.com and used again with very little modifications. These modifications are basically changing the icon's fonts location.
Is there certain criteria involved with using a public or purchased css file? Would any that claim to be a bootstrap based css file work?
I think that depends on the CSS author. Both Bootstrap CSS and themes from Bootswatch.com exists under the MIT license, and open source license which also allow us to use it in commercial projects, for example.
The better choice is to maintain all the required stuff in just one CSS file, just like the default Bootstrap CSS theme and also the Bootswatch.com includes themes. Other projects like "templates" can contains stuff we really don't need.
Which is higher on the list for final results, the css file or inlinecss? I assume the latter.
For little modifications, hacks, media queries (to react on different screen sizes) I think "Inline CSS" can be good. In fact there is a problem with that name. "Inline CSS" is not placed "inline" in the app's index.html file, but in a CSS file, just like if we include one of them. So we can use "Inline CSS" without space limitations or something like that.
If we plain, however, to modify a theme not only for one application, or just want to use certain complete CSS file(s), well, we can do it without problems. And therefore both options are not exclusives, then we can include CSS files and also write some specific "hacks" in the "Inline CSS" app's option.
Also it appears the Bootstrap customizer allows you to create your own css file. Is that how you created them? Can a current theme be uploaded and modified?
About the first question the answer is no: for App Builder I use the Bootstrap CSS the Bootswatch.com themes with very little modifications as I mentioned above. Honestly I don't know if we can upload a theme to be modified: probably not, since the tools works in the contrary way: they produces themes from certain code/variables, but probably the generator can't parse an already produced theme to obtain their sources/variables. Really I don't know.
I see there there are bootstrap themes available on the web. Most look like they are for websites, do they still work with the apps?
Yes, in principle, it's no reason to use it. However, CSS is very powerful, in the sense that CSS can change enterily the apparence of a control, etc. Then some themes can be more "usables" in applications. I think we can play with the fact that we can add CSS files as themes in the AB's themes directory, then we can take a look at the results very quickly. But a site like Bootswatch.com is appreciated, since they include in the website the final apparence of the available themes.
Also I did find the ones included as well. Very nice ones and would make a good link:
https://bootswatch.com/
Thanks
Yes; certainly we need certain "don" to produce these kind of stuff. I means that when I start a webpage, commonly, I start with "black and white" or maybe some "gray scale". Why? Because my "don" to play with colors, etc., is very limited, I recognize that. The guy beyond bootswatch.com can play with colors, etc., then can provide various useful themes ready to be used in our applications.