The dashboard Gramplets use the GrampletPane class, which in turn uses the grampletpane.glade file (in gramps.gui.glade subdir).
In that file there is a section that defines the “Textview”:
<child>
<object class="GtkTextView" id="gvtextview">
<property name="visible">True</property>
<property name="can_focus">True</property>
<property name="editable">False</property>
<property name="wrap_mode">word</property>
</object>
</child>
You would want to add some margins to that view, below the other properties:
<property name="left_margin">6</property>
<property name="right_margin">6</property>