1) my forum is intended to be a community on computer programming (in Thai language)

- can I use (integrate) "syntax hilighter" to highlight program codes in wpforo?

You should enable WordPress shortcodes in Dashboard > Forums > Settings > Features admin page. Then find some WordPress plugin for syntax highlighting.

2) the options to "like" and "unlike" belong to addon extension, do I understand correctly?

No, the like button is a free built in feature. There is no unlike button in wpForo. Only in Q&A layout you can vote up and down. This is also a free built in forum layout and feature.

3) you said that wpforo can combine with any wordpress theme. Could you please further illustrate this point? From my understanding, I can set wpforo style in its menu (eg changing color), but I dont understand how to combine with wp theme.

You should only change the color. There is no any action for theme combining. You have nothing to do here. Just install wpForo, choose the color style, and change the Primary color (#12) to closer to your theme. wpForo is automatically combined with all WordPress themes. It creates a page and loads all forum content in page with current theme wrapping design, header, footer, sidebars and widgets.

P.S. Please install wpForo and check all what you want. This is better than imagine based on my answers.

Thanks @robert and I apologise if my questions look liked I haven't install wpforo. I did and I am quite new to both wpforo and wordpress, so I just wasn't sure if there are some forum options/setting I could not find, or I misunderstood something.

I do think wpforo is a great forum, and I thank you so much for all your hard work!