Always use black text in PCB calculator E-series table

The white text in dark mode is too washed out on the cell colors to be
visible. Since we already purposely subdue the colors in dark mode, the
easiest fix is to just always use black text.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/17033

pcb_calculator/calculator_panels/panel_eseries_display.cpp

@@ -68,14 +68,18 @@ PANEL_ESERIES_DISPLAY::PANEL_ESERIES_DISPLAY( wxWindow * parent, wxWindowID id,
 
     // Make the grid lines disappear into the window background
     // making the value boxes appear to be separated from each other.
+    // Also force text to always be black so it is visible against the light
+    // colored cells in both light and dark modes.
     wxColour gridLineColour = parent->GetBackgroundColour();
 
+    m_GridEseries112->SetDefaultCellTextColour( *wxBLACK );
     m_GridEseries112->SetGridLineColour( gridLineColour );
     m_GridEseries112->EnableGridLines( true );
 
     m_GridEseries112->SetColLabelSize( wxGRID_AUTOSIZE );
     m_GridEseries112->AutoSize();
 
+    m_GridEseries2496->SetDefaultCellTextColour( *wxBLACK );
     m_GridEseries2496->SetGridLineColour( gridLineColour );
     m_GridEseries2496->EnableGridLines( true );