Page Layout Editor: View > Background White toggle breaks colour theme

Description

"Background White" toggle option in menubar choses the currently loaded theme.

If no one objects, I can submit a MR to remove this option since I believe the new colour theme stuff renders this obsolete.

Steps to reproduce

1. Open PL Editor
2. Load a non-default theme with a different background colour (e.g. Solarized Dark from https://github.com/pointhi/kicad-color-schemes)
3. Observe theme background colour.
4. Toggle View > Background White option in menubar.

KiCad Version

Application: PlEditor

Version: (5.99.0-2429-g55784afbf), debug build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.64.1 SecureTransport (LibreSSL/2.8.3) zlib/1.2.11 nghttp2/1.39.2

Platform: macOS Catalina Version 10.15.5 (Build 19F101), 64 bit, Little endian, wxMac

Build Info:
	Date: Jul 28 2020 16:44:21
	wxWidgets: 3.0.4 (wchar_t,STL containers,compatible with 2.8)
	Boost: 1.70.0
	OCE: 6.9.1
	Curl: 7.54.0
	ngspice: 28
	Compiler: Clang 11.0.0 with C++ ABI 1002

Build settings:
	KICAD_SCRIPTING=OFF
	KICAD_SCRIPTING_MODULES=OFF
	KICAD_SCRIPTING_PYTHON3=OFF
	KICAD_SCRIPTING_WXPYTHON=OFF
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
	KICAD_SCRIPTING_ACTION_MENU=OFF
	BUILD_GITHUB_PLUGIN=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE=OFF