Extended user customization

I've added some variables to extend user customization:

--cactus-editor-background-color
--cactus-comment-border-color
--cactus-comment-avatar-placeholder
--cactus-comment-margin-top
--cactus-comment-margin-bottom
--cactus-comment-gap

Users now can:

• Set the comment+name fields background with --cactus-editor-background-color
• Set the border-block-end color with --cactus-comment-border-color (defaults to .cactus-border-color)
• Set the character for .cactus-comment-avatar-placeholder::before with --cactus-comment-avatar-placeholder
• Set the comment general spacing with --cactus-comment-margin-top, --cactus-comment-margin-bottom, and --cactus-comment-gap

I've maintained all the previous defaults for each new variable.

I also removed a duplicate of .cactus-comment-avatar-placeholder::before.