Register two further "virtual" themes called BootstrapDark and BootstrapLight which force dark and light mode respectively. The actual Bootstrap theme itself will continue to auto-select dark mode preference based on OS/Browser preference settings. Fixes: #5492 Signed-off-by: Jo-Philipp Wich <jo@mein.io> (cherry picked from commit ed86f03a9f7c2f58a9111f925bb81303596b394b)