Hello,
I don't know if this module is still in development. I have started using it
last week. I tried to use LESS preprocessor, but it was not working. Module
created stylesheet in cache, yet it was not compiled. I investigated the module
code a bit and I found what was the reason. In ExaCSSBase class in Preprocessor
function there is a switch function which is used to choose an appropriate
preprocessor, such as was used in module settings. However there are two cases
for LESS precompiler (leafo and oyejorge), yet in module settings tab
'Parameters' there is just simple 'lessphp' option in the dropdown so it simply
doesn't work because there is no 'lessphp' case in the function switch. I used
one of two available compilers as 'lessphp' and module is now working. So I
suggest to update the preprocessors list in Parameters tab.
Best Regards, Tomek

P.S.
I am using 1.11.13 CMSMS Version, but there was no such option in Bug Report
dropdown.