Graphical glitch in multiline text boxes on dark themes

Description

Multiline text boxes have graphical glitches. These glitches go away when the text box is fully utilised, either horizontally or vertically. The glitches react at the text, see images:

image image image image

Also, the text box used to be and should be black with white text on dark theme. Maybe the graphical glitch used to be masked when the color settings were correct.

Steps to reproduce

  1. Open any dialog which includes multiline text box on system with dark theme in KiCAD. For example "Add text" dialog.
  2. Try writing in the box and observe the graphical glitch.

KiCad Version

Application: KiCad Schematic Editor

Version: 5.99.0-unknown-507ca9bc73~117~ubuntu20.04.1, release build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.68.0 OpenSSL/1.1.1f zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.2.0) libssh/0.9.3/openssl/zlib nghttp2/1.40.0 librtmp/2.3

Platform: Linux 5.4.0-65-generic x86_64, 64 bit, Little endian, wxGTK, mate, x11

Build Info:
	Date: Feb 23 2021 08:07:14
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCC: 7.3.0
	Curl: 7.68.0
	ngspice: 31
	Compiler: GCC 9.3.0 with C++ ABI 1013

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON