Simplified css color schemes

(cherry picked from commit 52b0bced5d235d7557413e064f3841ea774d9063)

_includes/css/just-the-docs.scss

+@import "./support/support";
+@import "./color_schemes/{{ include.color_scheme }}";
+@import "./modules";

_sass/modules.scss

@@ -17,3 +17,4 @@
 @import "./tables";
 @import "./code";
 @import "./utilities/utilities";
+@import "./custom/custom";

assets/css/just-the-docs-dark.scss

 ---
 ---
-@import "./support/support";
-@import "./color_schemes/dark";
-@import "./modules";
-@import "./custom/custom";
\ No newline at end of file
+{% include css/just-the-docs.scss color_scheme="dark" %}
\ No newline at end of file

assets/css/just-the-docs-light.scss

 ---
 ---
-@import "./support/support";
-@import "./color_schemes/light";
-@import "./modules";
-@import "./custom/custom";
+{% include css/just-the-docs.scss color_scheme="light" %}
\ No newline at end of file