I have tried as your explanation; but it seems to not work;

shall I know what is the meaning of "decoration-bf" and "decoration-muted"

decoration-bf: marks the record in bold font, decoration-muted: will mark it in gray, similarly there are quiet a few, basically they are used to indicate a quick status at glance.

I have changed my answer:

this exemple comes from the default sale module:

http://gitlab.opengest.solutions/opengest/odoo/blob/master/addons/sale/sale_view.xml#L114

As you can see, I've just added class="my_class"

In the same module, the sale's one, you can find an exemple how to insert custom CSS:

http://gitlab.opengest.solutions/opengest/odoo/blob/master/addons/sale/views/sale.xml

The css file:

http://gitlab.opengest.solutions/opengest/odoo/blob/master/addons/sale/static/src/css/sale.css

Thank you for you detail showing;

I have test this; and I find the css file can work even if delete "class="my_class" ", and I don;t know why you add this?

in addition, I want to the context in "xml" to be center when I use "text-align: center" in "css" file; and I find that the code can only make "fieds.Char" to be center rather than "fields.Float()".