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I want a "partial" style in Word 2019, such that when it's applied, the text only change some of its style settings, for example font or color, while keeping all other style settings, like line spacing or font size.

What I want to achieve exactly is like a new CSS rule, for example:

.code {

  font-family: Consolas;

  font-size: 16;

}

And then when I apply the "code" style to some existing text, they will all be changed into Consolas at size 16, while keeping their original colors, line spacing, etc., if they were previously different. This is exactly what happens when I add code class to an existing HTML element.

Is it possible with Word 2019?

You asked for it, so here it comes.

Word styles predate CSS by many years and are basically patch upon patch,
creating an enormous complexity. Most of us just keep on patching our documents
until they look correct. There has been times when I copied text from Word to
notepad, just to get rid of unwanted effects. So here are the details.

There is a big difference between applying a style to part-of or the entire
paragraph, called paragraph styles and character styles.
There is also a difference between direct and indirect styling, when style
is inherited from the paragraph or applied directly.

For a better understanding, read the article
Why does Word sometimes override bold and italics when I apply a paragraph style, but sometimes it does not?

When you apply a style to part of a paragraph, you create a linked style.
This style doesn't have a name but is yet modifiable and needs to be taken
into account, as weird as it sounds.

For details see the article
Linked Styles - Working with them in Microsoft Word.

See also the post
Word 2010: Create a Style without a font color.

If after reading the above you are still looking for the simplicity
of CSS (only relative simplicity since this is also a very complex subject),
then you should look for another text editor than Word.