Remove borders and paddings from native controls on Linux (#14873)

* init

* update

* Update SpinInput.cpp

* possible fix for em_unit

* button alignment

* match titlebar height

* revert em_value for macOS and Windows

* Update GUI_Utils.hpp

* Merge branch 'main' into linux-black-borders-2

* Revert "button alignment"

This reverts commit 3fc7461071.

* Revert "match titlebar height"

This reverts commit c4aa1d9f1e.

* revert dpi changes

* match platform tags

* remove radio box borders

* Fix code indent

src/slic3r/GUI/GUI_Utils.cpp

@@ -548,7 +548,7 @@ void RemoveButtonBorder(wxWindow* win)
     GtkCssProvider* provider = gtk_css_provider_new();
 
     const char* css =
-        "button {"
+        "button, button:hover, button:active, button:focus {"
         "  border: none;"
         "  outline: none;"
         "  box-shadow: none;"
@@ -589,6 +589,58 @@ void RemoveButtonBorder(wxWindow* win)
     );
 #endif
 }
+
+void RemoveInputBorder(wxWindow* win)
+{
+    GtkWidget* widget = win->GetHandle();
+    if (!widget) return;
+
+#if GTK_CHECK_VERSION(3, 0, 0)
+    // GTK3+: use CSS provider
+    GtkCssProvider* provider = gtk_css_provider_new();
+
+    // Target 'entry' and its inner subnodes (like text selection areas)
+    const char* css =
+        "entry, entry text, entry undershoot {"
+        "  border: none;"
+        "  outline: none;"
+        "  box-shadow: none;"
+        "  padding: 0px;"
+        "  margin: 0px;"
+        "  min-height: 0px;"
+        "  min-width: 0px;"
+        "  background: none;"
+        "}";
+
+#if GTK_CHECK_VERSION(4, 0, 0)
+    // GTK4
+    gtk_css_provider_load_from_data(provider, css, -1);
+#else
+    // GTK3
+    gtk_css_provider_load_from_data(provider, css, -1, nullptr);
+#endif
+
+    GtkStyleContext* ctx = gtk_widget_get_style_context(widget);
+    gtk_style_context_add_provider(
+        ctx,
+        GTK_STYLE_PROVIDER(provider),
+        GTK_STYLE_PROVIDER_PRIORITY_USER
+    );
+    g_object_unref(provider);
+
+#else
+    // GTK2: Target the x/y thickness of the entry widget
+    gtk_rc_parse_string(
+        "style \"no-padding-entry\" {"
+        "  xthickness = 0"
+        "  ythickness = 0"
+        "  GtkEntry::inner-border = { 0, 0, 0, 0 }"
+        "  GtkEntry::focus-line-width = 0"
+        "}"
+        "class \"GtkEntry\" style \"no-padding-entry\""
+    );
+#endif
+}
 #endif // __WXGTK__
 
 #ifdef __linux__