App bootstrap fails if system is set to dark and there is no dark theme

[kulmann]

Describe the bug

Given an ownCloud instance with only a light theme fails in app bootstrap if the operating system is set to dark mode.
(also happens the other way around, only a dark theme but system is set to light mode)

Steps to reproduce

1. Set up an ownCloud which only has a light theme
2. Set your operating system to dark mode
3. Visit your ownCloud instance in a private browser tab

Expected behavior

The ownCloud page you tried to open loads like it should, rendered with the only available theme.

Actual behavior

Missing or invalid config page is shown

Additional context

Happens on stable-8.0 (I didn't test if it also happens on master).
This is the line where it fails:

web/packages/web-pkg/src/composables/piniaStores/theme.ts

Line 107 in 4155b81

currentThemeName.value = unref(availableThemes).find((t) => t.isDark === unref(isDark)).name

[JammingBen]

Fixed via #10659.