Add theme: Zen0 Better Split Tab

[zen-browser-auto]

Add theme: Zen0 Better Split Tab

This PR adds a new theme to the theme library.

Theme Details

• Name: Zen0 Better Split Tab
• Description: Rework the Split Tab for a better overview
• Homepage: https://github.com/Ayamarre/Zen0-Better-Split-Tab
• Author: @Ayamarre

[Ayamarre]

/* >> Zen0 - Better Split Tab

• CSS for split tab with 2 tabs per row.

• Update : 2025-05-27 1.12.8b

• Author : Fluntek
  */

@-moz-document url-prefix("chrome:") {
tab-group[split-view-group] {
display: flex !important;
flex-wrap: wrap !important;
flex-direction: row !important;
align-content: flex-start !important;
padding: 2px !important;
overflow-y: auto !important;
gap: 2px !important;
}

tab-group[split-view-group] > .tabbrowser-tab {
flex: 1 1 calc(50% - 2px) !important;
min-width: 0 !important;
box-sizing: border-box !important;
min-height: 36px !important;
}

#tabbrowser-tabpanels[zen-split-view="true"] .browserSidebarContainer.deck-selected {
outline: var(--zen-colors-border) !important;
}
}