Repro:

main.cpp

#include <LibGUI/ConnectionToWindowServer.h>
#include <LibMain/Main.h>

ErrorOr<int> serenity_main(Main::Arguments)
{
    auto theme = GUI::ConnectionToWindowServer::the().get_system_theme();
    dbgln("Theme: {}", theme);
    return 0;
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.16)

project(Example)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)

set(SOURCES
    main.cpp
)

add_executable(Example ${SOURCES})
target_link_libraries(Example gui main)

Then run:

mkdir -p Build
cd Build
cmake .. -GNinja -DCMAKE_TOOLCHAIN_FILE="${SERENITY_SOURCE_DIR}/Build/${SERENITY_ARCH}/CMakeToolchain.txt"
ninja

Error:

[1/2] Building CXX object CMakeFiles/Example.dir/main.cpp.o
FAILED: CMakeFiles/Example.dir/main.cpp.o 
/home/linus/Dev/serenity/Toolchain/Local/x86_64/bin/x86_64-pc-serenity-g++ --sysroot=/home/linus/Dev/serenity/Build/superbuild-x86_64/../x86_64/Root   -std=c++20 -MD -MT CMakeFiles/Example.dir/main.cpp.o -MF CMakeFiles/Example.dir/main.cpp.o.d -o CMakeFiles/Example.dir/main.cpp.o -c /home/linus/Dev/example/main.cpp
In file included from /var/home/linus/Dev/example/main.cpp:1:
/home/linus/Dev/serenity/Build/x86_64/Root/usr/include/LibGUI/ConnectionToWindowServer.h:10:10: fatal error: WindowServer/ScreenLayout.h: No such file or directory
   10 | #include <WindowServer/ScreenLayout.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
ninja: build stopped: subcommand failed.

cc @ADKaster