via make.wordpress.org => original post link
To allow the setting of a custom opacity for each block, the Separator block has been updated to use the block that supports color settings. The custom opacity can then be set using the alpha channel setting of the selected color.
The HMTL structure of the block is unchanged, and all the existing classes are still in place, and two additional classes have been added – .has-alpha-channel-opacity and .has-css-opacity.
These new classes have been added to maintain the default 0.4 opacity for all existing blocks in both the editor and the frontend. The opacity for existing Separator blocks will only change if the block itself has its color setting changed. If theme authors have opted in to block styles with add_theme_support( ‘wp-block-styles’ ); and wish to maintain the default 0.4 opacity setting for both new and old blocks the following CSS can be added: