refs #23867 [finc] harmonise overlay colors

* add custom scss variable
* setting z-index of off-canvas overlay by own variable

co-authored by: Claas Kazzer <kazzer@uni-leipzig.de>
* adds Definitions and Variables for more flexible Offcanvas use
* adds missing variables
* corrects 'all content acts as toggler' error
* streamlines previous SCSS change

themes/finc/scss/components/_modal.scss

@@ -71,3 +71,8 @@
 .modal .table-resp-data td {
   word-break: break-word;
 }
+
+// set overlay-color
+#modal {
+  background-color: $overlay-color;
+}

themes/finc/scss/components/_offcanvas.scss

@@ -30,8 +30,6 @@
 }
 
 
-
-
 // *****************************************************************
 // ************ Off-canvas in Sidebar ******************************
 // *****************************************************************
@@ -82,7 +80,6 @@
 }
 
 
-
 // *****************************************************************
 // ************ Sidebar with offcanvas active **********************
 // *****************************************************************
@@ -101,6 +98,39 @@
 }
 
 
+// *****************************************************************
+// ************ Offcanvas Overlay **********************************
+// *****************************************************************
+.offcanvas-overlay {
+  // Overwrite BS Theme components/offcanvass.scss lines 32ff
+  // so definitions will be valid for tablet too
+  @media screen and (max-width: $offcanvas-default-max-width) {
+    background-color: $overlay-color;
+    cursor: pointer;
+    display: none;
+    height: 100%;
+    opacity: 0;
+    position: fixed;
+    top: 0;
+    transition: opacity .5s;
+    width: 100%;
+    z-index: $off-canvas-overlay-z-index;
+
+    .offcanvas.active & {
+      display: block;
+      opacity: 1;
+      transition: opacity .5s;
+    }
+
+    .offcanvas.active.offcanvas-left & {
+      right: -$offcanvas-offset;
+    }
+
+    .offcanvas.active.offcanvas-right & {
+      left: -$offcanvas-offset;
+    }
+  }
+}
 
 // *****************************************************************
 // ************ Tabs View in Result List ***************************