termguicolors vs :terminal: incorrect colors being displayed

I’m having trouble with the colors of the the terminal windown opened by :terminal.

I use a theme called PolarIce from themer.dev.

This work perfectly on KiTTY, Gnome Terminal and pretty much any terminal emulator I ever used, with or without Tmux.

However, the colors for the terminal window are off:

The above happens in both GVim and terminal Vim, with or without tmux.

Compared to an actual terminal window outside vim:

I have this in my .vimrc:

set termguicolors

Without this, the colorscheme does not look good, but the terminal colors matches the ones of the colorscheme.

I also have this in my .zshrc:

export TERM="screen-256color"

And this in my .tmux.conf:

set -g default-terminal 'screen-256color'
set -ga terminal-overrides ',screen-256color:Tc'

Here are my Vim installation details:

$ vim --version

VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Jun 06 2020 14:24:18)
  Correções incluídas: 1-914
  Compilado por Arch Linux
  Versão enorme com interface GTK3.  Recursos incluídos (+) ou não (-):
    +acl               -farsi             +mouse_sgr         +tag_binary
     +arabic            +file_in_path      -mouse_sysmouse    -tag_old_static
     +autocmd           +find_in_path      +mouse_urxvt       -tag_any_white
     +autochdir         +float             +mouse_xterm       +tcl/dyn
     -autoservername    +folding           +multi_byte        +termguicolors
     +balloon_eval      -footer            +multi_lang        +terminal
     +balloon_eval_term +fork()            -mzscheme          +terminfo
     +browse            +gettext           +netbeans_intg     +termresponse
     ++builtin_terms    -hangul_input      +num64             +textobjects
     +byte_offset       +iconv             +packages          +textprop
     +channel           +insert_expand     +path_extra        +timers
     +cindent           +ipv6              +perl/dyn          +title
     +clientserver      +job               +persistent_undo   +toolbar
     +clipboard         +jumplist          +popupwin          +user_commands
     +cmdline_compl     +keymap            +postscript        +vartabs
     +cmdline_hist      +lambda            +printer           +vertsplit
     +cmdline_info      +langmap           +profile           +virtualedit
     +comments          +libcall           +python/dyn        +visual
     +conceal           +linebreak         +python3/dyn       +visualextra
     +cryptv            +lispindent        +quickfix          +viminfo
     +cscope            +listcmds          +reltime           +vreplace
     +cursorbind        +localmap          +rightleft         +wildignore
     +cursorshape       +lua/dyn           +ruby/dyn          +wildmenu
     +dialog_con_gui    +menu              +scrollbind        +windows
     +diff              +mksession         +signs             +writebackup
     +digraphs          +modify_fname      +smartindent       +X11
     +dnd               +mouse             +sound             -xfontset
     -ebcdic            +mouseshape        +spell             +xim
     +emacs_tags        +mouse_dec         +startuptime       -xpm
     +eval              +mouse_gpm         +statusline        +xsmp_interact
     +ex_extra          -mouse_jsbterm     -sun_workshop      +xterm_clipboard
     +extra_search      +mouse_netterm     +syntax            -xterm_save
     arquivo vimrc de sistema: "/etc/vimrc"
     arquivo vimrc do usuário: "$HOME/.vimrc"
     2º arquivo vimrc do usuário: "~/.vim/vimrc"
     arquivo exrc do usuário: "$HOME/.exrc"
     arquivo gvimrc de sistema: "/etc/gvimrc"
     arquivo gvimrc do usuário: "$HOME/.gvimrc"
     2º arquivo gvimrc do usuário: "~/.vim/gvimrc"
     arquivo de padrões: "$VIMRUNTIME/defaults.vim"
     arquivo de menu do sistema: "$VIMRUNTIME/menu.vim"
     padrão para $VIM: "/usr/share/vim"
     Compilação: gcc -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_GTK  -I/usr/include/gtk-3.0 -I/usr/include/pango-1.0 -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/harfbuzz -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/fribidi -I/usr/include/uuid -I/usr/include/cairo -I/usr/include/pixman-1 -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/gio-unix-2.0 -I/usr/include/atk-1.0 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/dbus-1.0 -I/usr/lib/dbus-1.0/include -I/usr/include/at-spi-2.0 -pthread    -march=x86-64 -mtune=generic -O2 -pipe -fno-plt -D_REENTRANT  -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1
     Vinculação: gcc   -L. -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -fstack-protector-strong -rdynamic -Wl,-export-dynamic -Wl,-E -Wl,-rpath,/usr/lib/perl5/5.30/core_perl/CORE  -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -L/usr/local/lib -Wl,--as-needed -o vim   -lgtk-3 -lgdk-3 -lz -lpangocairo-1.0 -lpango-1.0 -lharfbuzz -latk-1.0 -lcairo-gobject -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0  -lSM -lICE -lXt -lX11 -lXdmcp -lSM -lICE  -lm -ltinfo -lelf    -lcanberra  -lacl -lattr -lgpm -ldl   -Wl,-E -Wl,-rpath,/usr/lib/perl5/5.30/core_perl/CORE -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -fstack-protector-strong -L/usr/local/lib  -L/usr/lib/perl5/5.30/core_perl/CORE -lperl -lpthread -ldl -lm -lcrypt -lutil -lc   -L/usr/lib -ltclstub8.6 -ldl -lz -lpthread -lm