Is it legally possible to obtain the css of the wordpress design template and incorporate it into other web applications?

In conclusion, Yes
(However, you may need your own customization)

2) Where should I look to decide these decisions?

Look at the license to make a decision

About Licenses

The WordPress theme inherits the principle GPL.However, the inheritance of GPL is required only for PHP, so depending on the paid theme, CSS and images may not be the same, so please check them at the time of purchase.
For example, GPLs are not limited in usage, so they can be used more than WordPress.

For example, all themes listed in the official directory are 100% GPL.

If you are not sure, please check with the seller or distributor.

example of themeforest

3) If I could successfully retrieve the design template, would it be possible to port it to a web application based on PHP, Ruby, Python, etc.?

I think this is a case by case...

The theme of WordPress is basically tailored to WordPress' spitting class.
In other words, it would be useful if you could create a WP site with that theme and build an application to spit out HTML similar to that site.

Depending on the theme, it may come with an original source that uses preprocessors such as SCSS, so it may work well.

Note: WP Theme Configuration

WP themes consist mainly of the following elements

• CSS (such as style.css)
• PHP functions for themes (files such as functions.php or /inc/template-tags.php)
• PHP Template
• JavaScript files (even if not)
• Image
• Translation files (out of theme these days...)

WP themes are mostly made for WP, so that's about...