A “solution” that causes crashes or works only with sample text isn’t a solution is it? If you have crashes, send them to Eastgate to determine the source and provide guidance.
The cleanest way to style text, markdown or HTML, is to embed the CSS code in the template for those notes.
All of these things are the function of the external command pointed to in $HTMLPreviewCommand, as discussed elsewhere. You can point to other parsers or executable scripts in /usr/local/bin/... rather than the default ```markdown.pl`` located in the Tinderbox app package. YMMV – Tinderbox is calling those external scripts as a convenience. Some may work, others will not.
As mentioned here in step #4.
CSS is a very advanced technique. Readers should be aware that merely copying CSS from another source (e.g., the Typora example at the head of this thread) may not work because CSS can invoke external code, including from the web, that Tinderbox has no ability to execute. I recommend working through the CSS examples at W3 Schools (free).