Will selling WP themes be a thing to invest in in the future?

The WP theme market is pretty saturated, there are still theme creators making money but it's more competitive that it used to be.

If you are skilled at WP development I would look at marketing plugins. Consider investing time in developing free plugins with paid add-ons. There is less competition as your plugin would fill a unique task/niche.

Sure there would be competitors and copy cats but it's much easier to provide a new feature/better support with plugins vs themes.

You could always test the water by creating a theme. Some are still doing really well.

I would say reusable HTML components (think widgets like carousels, maps) which allows users to customize CSS would be a better investment than building themes.

disclaimer: I'm building a new CMS called Pragma (https://pragma.build) which would allow users to compose pages by assembling components.

Difficult. Perhaps if you specialize, e.g. optimize for speed as much as possible by hosting assets on a CDN such that the buyer doesn't have to think about it. Most wordpress themes feel very slow.

But what do you mean by invest? Learning? If you can only do HTML & CSS you should perhaps search for themes in a market with more money... e.g. Shopify, Shopware

I don't think Wordpress will be going anywhere for quite some time however I also don't think it's market is expanding. I'm far from an expert on the matter but this is the impression I get from speaking with people here and in various development related Slack channels.

I think that we saw the peak of that with ThemeForest and those kind of markets but there will always be a market for something like WordPress.

Doubtful.