As i mention in another recent question though not new in programming, nor xojo (in fact programmer since REALBASIC), the bootstrap not solved yet, but, i question my self, how can i user change, for example, from a lite thema and back with the Bootstrap tecnnology?
Xojoās default Bootstrap Theme offer light and dark options.
Xojo by default set the Color Mode to Auto, so if you have your computer in Dark mode you will see the page in Dark mode, if set to Light, then Light.
With the Color Mode option you can put a checkbox (or something else) for your visitors to override the default and force light/dark/auto.
Most Bootstrap 5.x Themes that you download should offer Light/Dark mode (if the Default is not something you like).