(core) Selection for styled columns

Summary:
Grid UI improvements.
- Selecting a column with a custom background, didn't produce a visual difference.
- Hiding num-row right border when it is not needed (it should be used only for frozen columns)

Test Plan: manual tests

Reviewers: cyprien

Reviewed By: cyprien

Differential Revision: https://phab.getgrist.com/D3425

app/client/components/viewCommon.css

@@ -75,6 +75,10 @@
   color: var(--grist-actual-cell-color, black);
 }
 
+.field.selected .field_clip {
+  mix-blend-mode: darken;
+}
+
 .field_clip.invalid, .field_clip.field-error-from-style {
   background-color: #ffb6c1;
   color: black;