The encountered issue stems due to inheritance of CSS rules that select globally table elements. 

For the time being you can use the following CSS reset to improve the layout of the Editor's table elements:

.reLayoutWrapper, .reWrapper_corner, .reWrapper_center,
.reLeftVerticalSide, .reRightVerticalSide,
.reTlbVertical, .reToolCell, .reContentCell,
.reToolZone, .reBottomTable, .reEditorModesCell,
.reBottomZone, .reResizeCell {
	border: 0 none !important;
	padding: 0 !important;
}

.Telerik.reWrapper, .Telerik.RadEditor .reContentCell,
.Telerik.reColorPicker, .Telerik.reInsertTable,
.Telerik.reCustomLinks a:hover {
	border: 1px solid #828282 !important;
}

.reLeftVerticalSide, .reRightVerticalSide {
	padding: 1px !important;
}

Note that these CSS rules will affect RadEditor controls outside the RadGrid. If you need to modify only RadEditors inside RadGrid, use CssClass property and change the CSS selectors according the set class name.

Also, upon the used color you may need to change the border-color according to the Skin design.