How can I import my CSS style into a Java Application?
I have a normal CSS with the JButton#btn class styles and so on.
But how can I import it into my Application?
I want to turn the GUI with Java Swing with the file like in Python with the QT Framework
Swing doesn't work that way. To change the look of a Swing GUI, we set a Pluggable Look and Feel. It is true hat certain Swing components (including JButton) will render parts of HTML 3.2 and simple styles, and can indeed import stylesheets (and understand very basic CSS), it is implemented in a 'less than ideal' way. For example, if a button styled with HTML/CSS is disabled, it does not gain the default 'disabled look' for a button.
I am led to understand the Java-FX GUI toolkit allows more the style you are after. See the JavaFX CSS Reference Guide for details.
You don't import CSS into an application as a whole, CSS is applied directly to webpages. In order to insert an external CSS style into a page, you need to add this into <head> part
<link rel="stylesheet" type="text/css" href="mystyle.css">
You can check w3schools for more info about inserting CSS into a page.