So here some codes that might be beneficial for Sidebery or sidebar users in general:
Blending sidebar background with Zen theming scheme
(worked for default sidebars, but not with Sidebery)

#sidebar-box {
background-color: transparent;
color: var(--zen-colors-primary);
}
.sidebar-panel[lwt-sidebar] {
background-color: transparent;
}
Minimal sidebar title
(perfect if you use Sidebery panel as "workspaces" - check the "Use active panel's name as sidebar title" option in Sidebery setting first)
#sidebar-header {
overflow: hidden;
padding: 6px 0 0 0 !important;
font-size: 1em !important;
font-weight: bold !important;
border-bottom: 0 !important;
}
#sidebar-header #sidebar-switcher-target #sidebar-icon,
#sidebar-switcher-arrow,
#sidebar-close.close-icon,
#sidebar-splitter {
visibility: collapse !important;
}