Bootstrap Framework
Since Tiki 13 our Smarty templates changed to produce Bootstrap compatible HTML. Since Tiki 14 our theme system was completely rewritten so instead styles/ and separate templates/ folders all the themes reside in themes/ folder now.
Every developer should make sure that Tiki produces valid HTML5 and check if the structure is Bootstrap compatible.
Bootstrap 4 to 5 Migration Checklist
Here are some tools:
Bootlint
https://github.com/twbs/bootlint
Online tool: http://www.bootlint.com/