Fix multiple jobs build (build all libs before theme)

Trying to build theme while also building theme libs causes issues.
Now 'make -j 8' works, finally.
Closes #335

Makefile

@@ -156,7 +156,7 @@ interposer:
 plugin: backend bridges-plugin bridges-ui discovery
 	@$(MAKE) -C source/plugin
 
-theme:
+theme: libs
 	@$(MAKE) -C source/theme
 
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