[FIX] web: fix back/clear buttons in modal for small screen

This commit forces the color of all buttons in the header to have a
better-look consistency.

Steps to reproduce:
* Open Odoo on small screen
* Go to Sale
* Create a new SO
* Click on customer field to open the Kanban view => BUG
  the clear button is not visible (enterprise)

closes odoo/odoo#141040

Forward-port-of: #50123
X-original-commit: aa748820d06ddd84465c404ce158e882307dbda6
Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>
Signed-off-by: Romeo Fragomeli (rfr) <rfr@odoo.com>

addons/web/static/src/core/dialog/dialog.scss

@@ -51,7 +51,7 @@
                     .modal-header {
                         background: $o-navbar-background;
 
-                        .modal-title, button.close {
+                        .modal-title, .btn {
                             color: $o-navbar-entry-color;
                         }
                     }
@@ -88,20 +88,6 @@
                 align-items: center;
                 height: $o-navbar-height;
                 padding: 0 1rem;
-
-                .btn {
-                    color: white;
-                    background-color: transparent;
-                    border-color: transparent;
-
-                    &, .btn-sm {
-                        padding: 14px; // according to the padding for touch device
-                    }
-
-                    &.fa {
-                        padding: 10px; // according to the padding for touch device
-                    }
-                }
             }
 
             .modal-footer {