Fix documentation errors: wrong theme colors, missing variables, invalid HTML

[MADEVAL]

customizing.md:

• Fix 3 wrong color values in Default Oat brown section: danger (#df514c -> #d32f2f), success (#4caf50 -> #008032), warning (#ff8c00 -> #a65b00)
• Add missing --faint-foreground variable to Default Oat brown section
• Fix wrong color values in Theming reference section: primary, secondary-foreground, danger, success, warning, ring
• Remove non-existent --accent-foreground variable from Theming reference
• Fix dark mode docs: data-theme is not built-in, dark mode uses light-dark() + color-scheme automatically following OS preference

sidebar.md:

• Fix prose line 56: data-topnav goes on nav element, not header
• Fix structure table: data-topnav element is nav, not header
• Fix invalid HTML at line 33: removed duplicate class attribute, changed class=sm to class=small

button.md:

• Remove non-existent data-variant=primary from variant list. Default button without variant is already primary.

toast.md:

• Fix default variant in options table: empty string changed to 'info' (matches toast.js line 94)