Apply full RIVM styling to ro_gt_theme() #3

ro_gt_theme() only uses the RIVM color palette (ro_color()) once for the title text. Everything else falls back to hardcoded "black" or gt's own defaults (body text, column labels, row striping, borders).

To do

• Replace hardcoded color = "black" in column labels and subtitle (R/ro_gt_theme.R lines ~53, 61) with the appropriate RIVM color
• Give row group labels an explicit RIVM color (currently unset, falls back to gt default)
• Style row striping (opt_row_striping()) with an RIVM tint instead of gt's default stripe color
• Review borders/lines (column_labels.border.bottom.width, etc.) for RIVM-consistent color, not just width
• Cross-check against other functions like ro_gg_theme()/ro_e_theme() for which RIVM colors are used where, to keep styling consistent between graphs and tables