feat(theme): CherryBlossom atuin theme; document popup style

docs/terminal/tmux.mdx

@@ -67,6 +67,8 @@ The status line copies kitty's tab bar (`tab_bar_style powerline`, `tab_powerlin
 
 Colors are the user options `@tab-bg` (window background: the notch arrow and the session pill's text), `@tab-fg`, `@tab-active-bg`, and `@tab-active-fg`. They are a fixed CherryBlossom palette (the Noctalia custom palette in [prdlk/noctalia](https://github.com/prdlk/noctalia) `palettes/CherryBlossom.json`, the same one Ghostty's `cherry-blossom` theme uses), so the bar looks identical on every machine and does not depend on Omarchy. Active background = accent mixed 20% into background, the same derivation as the kitty tab bar: `#453130` from accent `#cf716f` over `#222120`.
 
+Popups (`prefix g`, `prefix t`, `prefix ?`, tmux-palette) set `popup-style` to `bg=@tab-bg` and `popup-border-style` to `@tab-active-fg` on that background with rounded lines. The explicit background matters under Ghostty: `background-opacity` only applies to cells with the default background, so a default-bg popup shows the pane behind it through the blur.
+
 ### Tab labels
 
 Each window reads `<index> <icon> <label>`, updating as you work, the same rule as kitty's `tab_bar.py`: