refactor: add backward compatibility for dark theme

Author: mrholek

scss/_functions.scss

@@ -318,11 +318,6 @@ $_luminance-list: .0008 .001 .0011 .0013 .0015 .0017 .002 .0022 .0025 .0027 .003
   @return if($return-calc == true, calc(#{$value1} - #{$value2}), $value1 + unquote(" - ") + $value2);
 }
 
-// Generate selectors for theme variants
-@function theme-prefix($selector) {
-  @return #{"." + $theme-prefix + "\\:" + $selector + ", " + "." + $selector + ":not([class*='#{$theme-prefix}:'])"};
-}
-
 @function divide($dividend, $divisor, $precision: 10) {
   $sign: if($dividend > 0 and $divisor > 0 or $dividend < 0 and $divisor < 0, 1, -1);
   $dividend: abs($dividend);
@@ -367,5 +362,5 @@ $_luminance-list: .0008 .001 .0011 .0013 .0015 .0017 .002 .0022 .0025 .0027 .003
 
 // Generate selectors for theme variants
 @function theme-prefix($theme, $selector) {
-  @return #{"[data-coreui-theme="$theme + "] ." + $theme + "\\:" + $selector + ", [data-coreui-theme="$theme + "] ." + $theme + "\\:" + $selector + ":not([class*='#{$theme}:'])"};
+  @return #{"[data-coreui-theme="$theme + "] ." + $theme + "\\:" + $selector + ",[data-coreui-theme="$theme + "] ." + $theme + "\\:" + $selector + ":not([class*='#{$theme}:'])", "." + $theme + "-theme ." + $theme + "\\:" + $selector + ", ." + $theme + "-theme ." + $theme + "\\:" + $selector + ":not([class*='#{$theme}:'])"};
 }

scss/mixins/_color-mode.scss

@@ -13,6 +13,7 @@
       }
     }
   } @else {
+    .#{$mode}-theme, // TODO: remove in v5
     [data-coreui-theme="#{$mode}"] {
       @content;
     }

scss/tests/mixins/_color-modes.test.scss

@@ -21,10 +21,12 @@
         }
       }
       @include expect() {
+        .dark-theme .element, // TODO: remove in v5
         [data-coreui-theme=dark] .element {
           color: var(--cui-primary-text-emphasis);
           background-color: var(--cui-primary-bg-subtle);
         }
+        .dark-theme, // TODO: remove in v5
         [data-coreui-theme=dark] {
           --custom-color: #3a3ff8;
         }