Can't get dotLESS #import working - css

Question

I'm struggling with the dotLESS #import to have a separate variables file; I just constantly get "variable is undefined".
If I browse to the variable config file it works; if I put the variables inline in the main stylesheet it works; but in an #import, no dice. I'm mapping .css as well as .less to the extension, however it also doesn't work if I use .less only.
The variables file LESS-config.less is:
/*
.LESS VARIABLES
*/
#mbw_dark_cyan: #1293b5;
#mbw_cyan: #11add4;
#mbw_magenta: #e935da;
#control_text: #ffffff;
#action_delete: #ff5400;
#section_level1_bg: #mbw_dark_cyan;
#section_level1_fg: #control_text;
#button_bg: #mbw_dark_cyan;
#button_fg: #control_text;
#button_icon: #control_text;
#data_table_header: #mbw_cyan;
.dummy {
color: #control_text;
}
Which renders as:
/*
.LESS VARIABLES
*/
.dummy {
color: #ffffff;
}
Calling stylesheet main.css is:
#import (less) '/css/LESS-config';
button {
background: #button_bg;
}
Which gives the error:
variable #button_bg is undefined on line 4 in file '/css/main.css':
[3]: button {
[4]: background: #button_bg;
----------------^
[5]: }
As I said, if I replace the import with the same variables copied and pasted, it all works fine.
I've tried saving without BOM as in another answer, but that doesn't help.
EDIT, I've tried:
Removing the (less)
Changing to double quotes
Using relative path LESS-config as opposed to virtual absolute as above
Adding logger="dotless.Core.Loggers.AspResponseLogger" log="debug" to
web.config (cache is already false)
Adding debug="1"
Adding
debug="true"
Absolutely no change in behaviour.
EDIT 2:
I created a cut-down css that only had the import statement in it; when I browse to it the imported styles are in there. However, on a refresh, I just get a blank response.
So it seems to be something to do with my IIS config / caching? I've turned off content compression but no joy; disabled all output caching for .less and .css, still no joy!
FIXED as per Toni's comment; https://stackoverflow.com/a/51754771/318411:
This turned out to be a dotLESS issue, tracked on GitHub here: https://github.com/dotless/dotless/issues/553
The complete fix was to:
Upgrade dotLESS to version 1.6.7
Downgrade Microsoft.Extensions.DependencyInjection to 1.1.1.0 due to Method
not found error
Change the file extension of the import from .css to .less
Now all working.

Answer 1

Please try version 1.6.7 which fixes an error that imports are only executed on the very first request.

Answer 2

I potentially see two problems that you have.
You are trying to call #import (less) in a css file. This is a syntax specific to less framework.
Your main.css is not a less file.
Change your main.css to a main.less file and now try generating your css from main.less as your root file.
Assuming your import url for LESS-config.less is correct.
The above mentioned corrections should probably do the trick.

Answer 3

#import (less, optional) "mystyle.css"; is Less syntax, you cannot use it in CSS (Less #import Rules).
If you want to use #import in your CSS, it should follow this syntax (See here)
#import url|string list-of-mediaqueries;
But, you cannot import a Less file inside your CSS anyways.
The way I would have done this:
Say you have 3 .less files: config.less, color.less, header.less
I would create a style.less file with the following content:
/*------------------------------------------------------------------------------*/
style.less
/*------------------------------------------------------------------------------*/
/* 01. config */
#import "config.less";
/* 02. color */
#import "color.less";
/* 03. header */
#import "header.less";
Then I would complie style.less which would produce, style.css and I would include style.css in my website.