Launching with a custom dark theme left every divider in light-theme colors until you re-toggled the theme in Settings.
`applyTheme` was copy-pasted across the components and the copies drifted. Three of them (+page.svelte's onMount block, AppLayout.svelte, NoteWindow.svelte) had no custom-* branch, so for a custom theme they stripped the dark class and data-theme attribute, matched nothing, and fell back to the bare light :root values.
`+layout.svelte` applied the custom theme correctly, but `AppLayout` — the always-mounted shell — re-runs its own broken copy from an `$effect` on `$theme`, immediately wiping it out. Dividers gave it away because custom themes override `--border-color` but not `--border-light`, which dividers use.
Fix: consolidate into a single `applyTheme`(which already owns darkThemes/isDarkTheme) with a real `custom-*` branch, and have every call site use it and pass the loaded custom themes so `custom-*` ids resolve.
Reviewed-on: https://codeberg.org/ArkHost/HelixNotes/pulls/200
