Fix: Dark/Light mode dialogs Find/Replace, Goto Line, and Find in Files dialogs

[electricmessiah]

• Dialog backgrounds (the space between controls) were never explicitly painted; the window class's null hbrBackground means DefWindowProc's default WM_ERASEBKGND handling is a no-op, so add an explicit handler that fills with the theme color (dark) or COLOR_BTNFACE (light).
• Checkbox (BS_AUTOCHECKBOX) label text ignored WM_CTLCOLORBTN's color while visual styles were active, rendering black-on-dark and unreadable; strip visual styles on checkboxes in dark mode (reversible) so the color takes effect.
• All theming (titlebar, child-control theme, checkbox visual style) was only ever applied once at dialog creation. Since these dialogs are cached (shown/hidden, not recreated), toggling dark mode after a dialog was already open left it permanently on the stale theme, and once dark had disabled a checkbox's visual style, going back to light never restored it. Re-apply full theming on every dialog show and immediately when dark mode is toggled while a dialog is already open.
• Fixed a pre-existing layout bug surfaced by the above: the Wrap around checkbox's bounding box (x=330..410) overlapped the button column (x=360..450) by 50px in the Find/Replace dialog; widened the dialog and shifted the button column clear of it.