Style Active Button (Premium)
To style the active button, you can use the df-activebutton class, that gets automatically added to the active button.
For example:
This code makes the active button darker
.df-activebutton {
filter: brightness(50%);

Have the "Show all" button styled after page load

To have the all button already styled after page load, just add the df-activebutton class to the button.

This is a Premium feature

Make sure to read how to get the Premium features in the following article:

Where to add custom CSS?

This you can find on the following article page: