[Refactor] Tidy up utils' files and functions.

src-ui/app/_index_css/root.css

@@ -0,0 +1,42 @@
+@import "./reset.css";
+@import "./variables.css";
+
+:root {
+    font-size: 62.5%;
+    color: #F2F2F2;
+    font-family: "Yu Gothic UI";
+}
+
+* {
+    user-select: none;
+    &::-webkit-scrollbar {
+        width: 0.8rem;
+    }
+    &::-webkit-scrollbar-track {
+        background-color: var(--dark_925_color);
+        border-radius: 0.4rem;
+    }
+    &::-webkit-scrollbar-thumb {
+        background-color: var(--dark_800_color);
+        border-radius: 0.4rem;
+    }
+}
+
+html, body {
+    height: 100%;
+    font-family: "Yu Gothic UI"; /* If not found the font family where 'root:' that is selected by user*/
+    border-radius: 1.8rem;
+}
+
+
+#root {
+    height: 100%;
+
+    /* For controlling a whole window transparency */
+    opacity: 1;
+}
+
+/* SVG内のすべての要素にfillを適用 (colorの調整をcssでするため) */
+svg {
+    fill: currentColor;
+}