diff --git a/packages/ckeditor5-list/theme/todolist.css b/packages/ckeditor5-list/theme/todolist.css
index fee48c7bd68..36cd7840779 100644
--- a/packages/ckeditor5-list/theme/todolist.css
+++ b/packages/ckeditor5-list/theme/todolist.css
@@ -25,7 +25,7 @@
 	margin-left: 0;
 
 	/* RTL styles */
-	@nest [dir=rtl]& {
+	[dir=rtl]& {
 		left: 0;
 		margin-right: 0;
 		right: -25px;
diff --git a/packages/ckeditor5-theme-lark/theme/ckeditor5-html-embed/htmlembed.css b/packages/ckeditor5-theme-lark/theme/ckeditor5-html-embed/htmlembed.css
index 2247d4bfe04..a25a758fc6f 100644
--- a/packages/ckeditor5-theme-lark/theme/ckeditor5-html-embed/htmlembed.css
+++ b/packages/ckeditor5-theme-lark/theme/ckeditor5-html-embed/htmlembed.css
@@ -56,18 +56,18 @@
 		margin-left: 50px;
 	}
 
-	@nest .ck.ck-editor__editable.ck-blurred &.ck-widget_selected::before {
+	.ck.ck-editor__editable.ck-blurred &.ck-widget_selected::before {
 		top: 0px;
 		padding: var(--ck-spacing-tiny) var(--ck-spacing-small);
 	}
 
-	@nest .ck.ck-editor__editable:not(.ck-blurred) &.ck-widget_selected::before {
+	.ck.ck-editor__editable:not(.ck-blurred) &.ck-widget_selected::before {
 		top: 0;
 		padding: var(--ck-spacing-tiny) var(--ck-spacing-small);
 		background: var(--ck-color-focus-border);
 	}
 
-	@nest .ck.ck-editor__editable &:not(.ck-widget_selected):hover::before {
+	.ck.ck-editor__editable &:not(.ck-widget_selected):hover::before {
 		top: 0px;
 		padding: var(--ck-spacing-tiny) var(--ck-spacing-small);
 	}
@@ -135,7 +135,7 @@
 		width: var(--ck-html-embed-content-width);
 
 		/* Disable all mouse interaction as long as the editor is not read–only. */
-		@nest .ck-editor__editable:not(.ck-read-only) & {
+		.ck-editor__editable:not(.ck-read-only) & {
 			pointer-events: none;
 		}
 	}
diff --git a/packages/ckeditor5-theme-lark/theme/ckeditor5-ui/components/dropdown/splitbutton.css b/packages/ckeditor5-theme-lark/theme/ckeditor5-ui/components/dropdown/splitbutton.css
index 56db783b458..3b5f27759c9 100644
--- a/packages/ckeditor5-theme-lark/theme/ckeditor5-ui/components/dropdown/splitbutton.css
+++ b/packages/ckeditor5-theme-lark/theme/ckeditor5-ui/components/dropdown/splitbutton.css
@@ -16,13 +16,13 @@
 	 */
 	&:hover > .ck-splitbutton__action,
 	&.ck-splitbutton_open > .ck-splitbutton__action {
-		@nest [dir="ltr"] & {
+		[dir="ltr"] & {
 			/* Don't round the action button on the right side */
 			border-top-right-radius: unset;
 			border-bottom-right-radius: unset;
 		}
 
-		@nest [dir="rtl"] & {
+		[dir="rtl"] & {
 			/* Don't round the action button on the left side */
 			border-top-left-radius: unset;
 			border-bottom-left-radius: unset;
@@ -34,13 +34,13 @@
 		it must get some arbitrary min-width. */
 		min-width: unset;
 
-		@nest [dir="ltr"] & {
+		[dir="ltr"] & {
 			/* Don't round the arrow button on the left side */
 			border-top-left-radius: unset;
 			border-bottom-left-radius: unset;
 		}
 
-		@nest [dir="rtl"] & {
+		[dir="rtl"] & {
 			/* Don't round the arrow button on the right side */
 			border-top-right-radius: unset;
 			border-bottom-right-radius: unset;
@@ -58,6 +58,20 @@
 		border-bottom-width: 0px;
 	}
 
+	/* Don't round the bottom left and right corners of the buttons when "open"
+	https://github.com/ckeditor/ckeditor5/issues/816 */
+	&.ck-splitbutton_open {
+		@mixin ck-rounded-corners {
+			& > .ck-splitbutton__action {
+				border-bottom-left-radius: 0;
+			}
+
+			& > .ck-splitbutton__arrow {
+				border-bottom-right-radius: 0;
+			}
+		}
+	}
+
 	/* When the split button is "open" (the arrow is on) or being hovered, it should get some styling
 	as a whole. The background of both buttons should stand out and there should be a visual
 	separation between both buttons. */
@@ -83,30 +97,16 @@
 			--ck-color-split-button-hover-border: var(--ck-color-focus-border);
 		}
 
-		@nest [dir="ltr"] & {
+		[dir="ltr"] & {
 			& > .ck-splitbutton__arrow:not(.ck-disabled)::after {
 				left: -1px;
 			}
 		}
 
-		@nest [dir="rtl"] & {
+		[dir="rtl"] & {
 			& > .ck-splitbutton__arrow:not(.ck-disabled)::after {
 				right: -1px;
 			}
 		}
 	}
-
-	/* Don't round the bottom left and right corners of the buttons when "open"
-	https://github.com/ckeditor/ckeditor5/issues/816 */
-	&.ck-splitbutton_open {
-		@mixin ck-rounded-corners {
-			& > .ck-splitbutton__action {
-				border-bottom-left-radius: 0;
-			}
-
-			& > .ck-splitbutton__arrow {
-				border-bottom-right-radius: 0;
-			}
-		}
-	}
 }
diff --git a/packages/ckeditor5-theme-lark/theme/ckeditor5-ui/components/toolbar/toolbar.css b/packages/ckeditor5-theme-lark/theme/ckeditor5-ui/components/toolbar/toolbar.css
index 2e4d8ec9db5..e77890bddc1 100644
--- a/packages/ckeditor5-theme-lark/theme/ckeditor5-ui/components/toolbar/toolbar.css
+++ b/packages/ckeditor5-theme-lark/theme/ckeditor5-ui/components/toolbar/toolbar.css
@@ -110,7 +110,7 @@
 		outline: none;
 	}
 
-	@nest .ck-toolbar-container & {
+	.ck-toolbar-container & {
 		border: 0;
 	}
 }
diff --git a/packages/ckeditor5-theme-lark/theme/mixins/_rounded.css b/packages/ckeditor5-theme-lark/theme/mixins/_rounded.css
index 3d260a90bcd..923e9b99d27 100644
--- a/packages/ckeditor5-theme-lark/theme/mixins/_rounded.css
+++ b/packages/ckeditor5-theme-lark/theme/mixins/_rounded.css
@@ -11,7 +11,7 @@
 @define-mixin ck-rounded-corners {
 	border-radius: 0;
 
-	@nest .ck-rounded-corners &,
+	.ck-rounded-corners &,
 	&.ck-rounded-corners {
 		border-radius: var(--ck-border-radius);
 		@mixin-content;
diff --git a/packages/ckeditor5-ui/theme/components/dialog/dialog.css b/packages/ckeditor5-ui/theme/components/dialog/dialog.css
index 452dce25bfb..657fea5ca8b 100644
--- a/packages/ckeditor5-ui/theme/components/dialog/dialog.css
+++ b/packages/ckeditor5-ui/theme/components/dialog/dialog.css
@@ -33,7 +33,7 @@
 		}
 	}
 
-	@nest .ck.ck-dialog-overlay.ck-dialog-overlay__transparent & {
+	.ck.ck-dialog-overlay.ck-dialog-overlay__transparent & {
 		pointer-events: all;
 	}
 }
diff --git a/packages/ckeditor5-ui/theme/mixins/_dir.css b/packages/ckeditor5-ui/theme/mixins/_dir.css
index 21a61587ca2..513785c06fa 100644
--- a/packages/ckeditor5-ui/theme/mixins/_dir.css
+++ b/packages/ckeditor5-ui/theme/mixins/_dir.css
@@ -4,7 +4,7 @@
  */
 
 @define-mixin ck-dir $direction {
-	@nest [dir="$(direction)"] & {
+	[dir="$(direction)"] & {
 		@mixin-content;
 	}
 }