Fix indeterminate input background color (#1536) (#1548)

src/scss/_variables.scss

@@ -850,6 +850,7 @@ $form-check-input-checked-bg-repeat: repeat !default;
 $form-check-input-checked-bg-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' width='16' height='16'><path fill='none' stroke='#{$white}' stroke-linecap='round' stroke-linejoin='round' stroke-width='2' d='M4 8.5l2.5 2.5l5.5 -5.5'/></svg>") !default;
 $form-check-input-checked-bg-image-dark: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' width='16' height='16'><path fill='none' stroke='#{$body-color}' stroke-linecap='round' stroke-linejoin='round' stroke-width='2' d='M4 8.5l2.5 2.5l5.5 -5.5'/></svg>") !default;
 $form-check-input-checked-border-color: $input-border-color-translucent !default;
+$form-check-input-indeterminate-bg-color: var(--#{$prefix}primary) !default;
 
 $form-check-radio-checked-bg-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16'><circle r='3' fill='#{$white}' cx='8' cy='8' /></svg>") !default;