docs(grid): sync JP styling section updates to match EN samples

Updated SCSS code blocks in JP hierarchical grid and tree grid docs
to match the simplified styling approach used in the English source.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

docs/angular/src/content/jp/components/hierarchicalgrid/hierarchical-grid.mdx

@@ -343,17 +343,14 @@ igxHierarchicalGrid を使用すると、[`Ignite UI for Angular テーマ ラ
  </DocsAside>
 
 ```scss
-$custom-theme: grid-theme(
-  $cell-active-border-color: #ffcd0f,
-  $cell-selected-background: #6f6f6f,
-  $row-hover-background: #f8e495,
-  $row-selected-background: #8d8d8d,
-  $header-background: #494949,
-  $header-text-color: #fff,
-  $expand-icon-color: #ffcd0f,
-  $expand-icon-hover-color: #e0b710,
-  $resize-line-color: #ffcd0f,
-  $row-highlight: #ffcd0f
+$background: #292826;
+$foreground: #eeece1;
+$accent: #ffcd0f;
+
+$custom-grid: grid-theme(
+  $background: $background,
+  $foreground: $foreground,
+  $accent-color: $accent,
 );
 ```
 
@@ -367,7 +364,7 @@ $custom-theme: grid-theme(
 
 ```scss
 :host {
-  @include tokens($custom-theme);
+  @include tokens($custom-grid);
 }
 ```
 

docs/angular/src/content/jp/components/treegrid/tree-grid.mdx

@@ -303,17 +303,14 @@ Tree Grid は、[`Ignite UI for Angular テーマ ライブラリ`](/themes/sass
 </DocsAside>
 
 ```scss
-$custom-theme: grid-theme(
-  $cell-active-border-color: #ffcd0f,
-  $cell-selected-background: #6f6f6f,
-  $row-hover-background: #f8e495,
-  $row-selected-background: #8d8d8d,
-  $header-background: #494949,
-  $header-text-color: #fff,
-  $expand-icon-color: #ffcd0f,
-  $expand-icon-hover-color: #e0b710,
-  $resize-line-color: #ffcd0f,
-  $row-highlight: #ffcd0f
+$background: #292826;
+$foreground: #eeece1;
+$accent: #ffcd0f;
+
+$custom-grid: grid-theme(
+  $background: $background,
+  $foreground: $foreground,
+  $accent-color: $accent,
 );
 ```
 
@@ -325,7 +322,7 @@ $custom-theme: grid-theme(
 
 ```scss
 :host {
-  @include tokens($custom-theme);
+  @include tokens($custom-grid);
 }
 ```