Ask HN: How can a person with web dev skills make $500-$1000 per month?

Yes. Also your English is fine, assuming this is a representative sample.

> considering the present reality of code-on-whiteboard sort of interviews for a stable job in the major software companies

My advice here is that it's a coder's market and you can mostly redirect these into other methods of demonstrating your competency, or if nothing else - iterate over several companies until you find one with more realistic methods.

> What do you think about passive income, say, from selling WordPress themes on themeforest, or something similar?

"Passive" income is mostly a misnomer. It's more a matter of, "I have a mostly post-development product, which occasionally requires maintenance, and the active portion of monetizing it is now more a matter of sales and marketing, versus engineering."

More generally:

The set of companies hiring or expecting to hire software engineers is approximately, "all companies which presently employ software engineers." The set of companies which can benefit from software engineering is approximately, "all companies with meaningful revenue."

The part that remains is for you to find them and convince them that particular things you can do with code will predictably lead to them having more revenue. This is easier if you specialize in a particular type of company and problem, because you can build up knowledge around what problems they have, how they currently deal with them, how they talk about them, and what sales and marketing approaches they're most receptive to.

This applies whether you want to be an employee, or sell a product, or provide consulting services, or whatever. Find people who have problems, convince them that you can provide solutions in a way which is predictably revenue-positive and which doesn't unduly increase the burden to them.

Your English looks flawless to me, by the way, so don't sell yourself short: you can learn to write like a pro, you may already be there, and describing what you have done and what you are going to do in written English prose is where the money is.

Are you actually a Wordpress themer? Learn just that much PHP and you can get all kinds of little gigs, doing work like customizing themes for people's Wordpress or Drupal sites. The advantage of learning a package, like a CMS, is that they lend themselves to little jobs, because most of their code is not customized and oftentimes the custom code is, shall we say, easy to improve. And if you are looking to fill time for a few months - and practice those vital pitching and customer-communication skills - little jobs might be nice.

Incidentally, at some point you might really want to practice whiteboard interviews, but if I were to do that I would specifically practice whiteboard interviews, not TopCoder or algorithms. Whiteboard interviews are a specific skill all their own. That skill is at least as much about being able to think out loud as it is about what you are actually thinking about. Knowing more algorithms might not help. I have found it more valuable to be able to implement bubble sort while telling jokes about bubble sort than to know what quicksort is.