(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/GridView.css

@@ -235,7 +235,6 @@
   width: 0px; /* Matches rowid width (+border) */
   height: 100%;
   z-index: 3;
-  left: calc(4rem);
   border-right: 1px solid var(--grist-color-dark-grey) !important;
   user-select: none;
   pointer-events: none