Makefile foreach compiling themes - foreach

Question

I'm currently trying to compile all my themes in one go, but I can't seem to get it to work.
E.g.:
THEMES := theme_a theme_b theme_c theme_d
build_themes:
$(foreach THEME, $(THEMES), sass web/themes/$(THEME)/scss/style.scss web/themes/$(THEME)/css/style.css)
But it just compiles the first theme and not all.
Is there something wrong with my iteration?
EDIT
After the suggestion by Renaud Pacalet:
RUBY =$(whereis ruby)
SASSC = $(RUBY) sass --style compressed
THEMES := theme_a theme_b theme_c theme_d
SASSDIR = build/scss
CSSDIR = web/themes/custom-themes
SASS := $(wildcard $(SASSDIR)/style_*.scss)
CSS := $(patsubst $(SASSDIR)/style_%.scss, $(CSSDIR)/%/css/style.css, $(SASS))
all: $(CSS)
$(CSSDIR)/%/css/style.css: $(SASSDIR)/style_%.scss
$(SASSC) $< $#
But I still can't seem to get it to work

Answer 1

Make is not yet another scripting language. It is a dependency management utility with building capability. Tell it how to compile any theme:
web/themes/%/css/style.css: web/themes/%/scss/style.scss
sass $< $#
Note that the recipe line (sass $# $<) starts with a tab, not spaces.
This is a pattern rule. $< and $# are two automatic variables that make will substitute respectively by the first prerequisite of the rule and by its target.
This rule means that any CCS named web/themes/<something>/css/style.css depends on the (only) prerequisite web/themes/<something>/scss/style.scss. If make is told to build a CSS, it will search for the corresponding SCSS. If it does not find the SCSS, it will raise an error. If it finds it, it will compare the last modification dates of the CSS and SCSS. If the CSS is newer than the SCSS, make will consider that the CSS is up-to-date and it will not do anything. Else it will consider that the CSS is outdated and it will expand the recipe (by substituting $# and $<) and pass it to the shell.
Next, tell make that you want all your themes to be compiled:
THEMES := theme_a theme_b theme_c theme_d
CSS := $(patsubst %,web/themes/%/css/style.css,$(THEMES))
all: $(CSS)
And that's it. Just invoke make all to get the work done.
The most important thing to remember here is that, thanks to this structure:
target: prerequisite
recipe
make will do much better than a naive script loop: it will re-compile a theme only if the CSS is missing or older than the corresponding SCSS. If invoked with the -j8 option (make -j8 all) it will even parallelize the work by launching up to 8 jobs in parallel.
Bonus: make can also find all your SCSS and compute the list of all buildable CSS:
DIR := web/themes
SCSS := $(wildcard $(DIR)/*/scss/style.scss)
CSS := $(patsubst $(DIR)/%/scss/style.scss,$(DIR)/%/css/style.css,$(SCSS))
all: $(CSS)
$(DIR)/%/css/style.css: $(DIR)/%/scss/style.scss
sass $< $#