...
add_custom_command(
    OUTPUT ${Breeze_TARGETS}
    DEPENDS ${SOURCES}
    COMMAND ./build_theme.sh -c Breeze -t "${PROJECT_BINARY_DIR}/Breeze" -r "${BREEZE_COLOR_INSTALL_ROOT}"
    COMMAND cp gtk-dark-3.0.css "${PROJECT_BINARY_DIR}/Breeze/gtk-3.0/gtk-dark.css"
    WORKING_DIRECTORY "${PROJECT_SOURCE_DIR}/src"
)
...

...
# Usage: build_sass <source-directory> <target-directory> <include-directory>
build_sass() {
  if command -v sassc >/dev/null 2>&1; then
      sassc -I "$3" "$1" "$2"
  else
      sass -I "$3" --cache-location /tmp/sass-cache "$1" "$2"
  fi
}
...

$ sassc -v
sassc: na
libsass: na
sass2scss: 1.1.1
sass: 3.5

sassc -v
sassc: na
libsass: na
sass2scss: 1.1.1
sass: 3.5

(gdb) bt
#0  0x00007f59714be861 in Sass::SharedPtr::incRefCount (this=0x7fffc1a2ba08) at memory/SharedPtr.hpp:174
#1  Sass::SharedPtr::SharedPtr (ptr=0x556ce0e3b660, this=0x7fffc1a2ba08) at memory/SharedPtr.hpp:114
#2  Sass::SharedPtr::SharedPtr (obj=..., this=0x7fffc1a2ba08) at memory/SharedPtr.hpp:116
#3  Sass::SharedImpl<Sass::SelectorComponent>::SharedImpl (this=0x7fffc1a2ba08) at memory/SharedPtr.hpp:183
#4  Sass::ComplexSelector::minSpecificity (this=this@entry=0x556ce0e60820) at ast_selectors.cpp:843
#5  0x00007f5971593662 in Sass::Extender::dontTrimComplex (complex2=0x556ce0e60820, complex1=0x556ce0e60820, maxSpecificity=2002) at extender.cpp:1163
#6  0x00007f597159390b in Sass::hasAny<std::vector<Sass::SharedImpl<Sass::ComplexSelector>, std::allocator<Sass::SharedImpl<Sass::ComplexSelector> > >, bool (*)(Sass::ComplexSelector const*, Sass::ComplexSelector const*, unsigned long), Sass::SharedImpl<Sass::ComplexSelector>, unsigned long> (
    fn=0x7f5971593650 <Sass::Extender::dontTrimComplex(Sass::ComplexSelector const*, Sass::ComplexSelector const*, unsigned long)>, 
    cnt=std::vector of length 38733, capacity 65536 = {...}) at memory/SharedPtr.hpp:215
#7  Sass::Extender::trim (this=0x556ce0a66f88, selectors=std::vector of length 77, capacity 128 = {...}, 
    existing=std::unordered_set with 187 elements = {...}) at extender.cpp:1109
#8  0x00007f5971593c7d in Sass::Extender::extendList (this=0x556ce0a66f88, list=..., extensions=std::unordered_map with 6 elements = {...}, 
    mediaQueryContext=...) at memory/SharedPtr.hpp:147
#9  0x00007f59715984dc in Sass::Extender::addSelector (this=0x556ce0a66f88, selector=..., mediaContext=...) at extender.cpp:186
#10 0x00007f5971585674 in Sass::Expand::operator() (this=0x7fffc1a2ca70, r=0x556ce0b31fe0)
    at /usr/x86_64-pc-linux-gnu/include/c++/9.2.0/bits/stl_iterator.h:867
#11 0x00007f5971587d93 in Sass::Expand::append_block (this=this@entry=0x7fffc1a2ca70, b=b@entry=0x556ce0b319e0) at expand.cpp:846
#12 0x00007f5971580e05 in Sass::Expand::operator() (this=0x7fffc1a2ca70, b=0x556ce0b319e0) at expand.cpp:140
#13 0x00007f59715856bf in Sass::Expand::operator() (this=0x7fffc1a2ca70, r=0x556ce0b31450) at memory/SharedPtr.hpp:175
#14 0x00007f5971587d93 in Sass::Expand::append_block (this=this@entry=0x7fffc1a2ca70, b=b@entry=0x556ce0b33e60) at expand.cpp:846
#15 0x00007f5971580e05 in Sass::Expand::operator() (this=0x7fffc1a2ca70, b=0x556ce0b33e60) at expand.cpp:140
#16 0x00007f597157f05a in Sass::Expand::operator() (this=0x7fffc1a2ca70, a=0x556ce0b33f20) at memory/SharedPtr.hpp:147
#17 0x00007f5971587d93 in Sass::Expand::append_block (this=this@entry=0x7fffc1a2ca70, b=b@entry=0x556ce0b30bd0) at expand.cpp:846
#18 0x00007f5971580e05 in Sass::Expand::operator() (this=0x7fffc1a2ca70, b=0x556ce0b30bd0) at expand.cpp:140
#19 0x00007f59715856bf in Sass::Expand::operator() (this=0x7fffc1a2ca70, r=0x556ce0b30810) at memory/SharedPtr.hpp:175
#20 0x00007f5971587d93 in Sass::Expand::append_block (this=this@entry=0x7fffc1a2ca70, b=0x556ce0b03550) at expand.cpp:846
#21 0x00007f5971584486 in Sass::Expand::operator() (this=0x7fffc1a2ca70, i=<optimized out>) at memory/SharedPtr.hpp:215
#22 0x00007f5971587d93 in Sass::Expand::append_block (this=this@entry=0x7fffc1a2ca70, b=b@entry=0x556ce0a68070) at expand.cpp:846
#23 0x00007f5971580e05 in Sass::Expand::operator() (this=this@entry=0x7fffc1a2ca70, b=0x556ce0a68070) at expand.cpp:140
#24 0x00007f59714cbda2 in Sass::Context::compile (this=0x556ce0a66d10) at memory/SharedPtr.hpp:215
#25 0x00007f59714ce0c9 in Sass::File_Context::parse (this=0x556ce0a66d10) at context.cpp:579
#26 0x00007f59715b2c57 in Sass::sass_parse_block (compiler=0x556ce0a67690) at sass_context.cpp:180
#27 sass_compiler_parse (compiler=0x556ce0a67690) at sass_context.cpp:434
#28 sass_compiler_parse (compiler=0x556ce0a67690) at sass_context.cpp:424
#29 0x00007f59715b3059 in sass_compile_context (c_ctx=0x556ce0a66bc0, cpp_ctx=<optimized out>) at sass_context.cpp:317
#30 0x0000556cdf940616 in compile_file ()
#31 0x0000556cdf9400dd in main ()