Update declarative_styles.rst: Update code example to attrs TNG usage. (#8072)

Update declarative_styles.rst: Update code example to attrs TNG usage. (#8072)

* Update declarative_styles.rst

Update docs to new style usage of attrs.
This is the default since December 2021.
While the old style still works, the newer one looks much nicer and is likely to be dominant pretty quickly. Imho.

* Update declarative_styles.rst

* Update declarative_styles.rst

(cherry picked from commit 1e1431fbfb4992f8d85d3fd42ec06975e7f762f0)