[FIX] web_editor, website: color-combination preview misleading design

Adjust shadows/borders in order to make the color-combination box
stand-up.

task-2335267
part-of: #57942

addons/web_editor/static/src/scss/wysiwyg_snippets.scss

@@ -1328,6 +1328,7 @@ body.editor_enable.editor_has_snippets {
             padding: 0;
             margin: 0;
             border: 0;
+            background-color: transparent;
             background-clip: padding-box;
             border-top: $o-we-sidebar-content-field-spacing solid transparent;
             border-bottom: $o-we-sidebar-content-field-spacing solid transparent;

addons/website/static/src/scss/website.wysiwyg.scss

@@ -172,7 +172,11 @@ $i: 1;
 
     // CC Edition
     .o_we_cc_preview_wrapper {
-        border: 1px solid $o-we-sidebar-content-field-dropdown-border-color;
+        // Use box-shadow rather than border-bottom in order to
+        // avoid misalignments in the 'Options' tab.
+        border: 1px solid;
+        border-color: rgba($o-we-item-standup-color-light, .2) $o-we-sidebar-content-field-dropdown-border-color transparent;
+        box-shadow: 0 1px 0 $o-we-item-standup-color-dark;
 
         + .o_we_collapse_toggler {
             height: 35px; // FIXME hardcoded...