Function vga_debug_fill_line 

unsafe fn vga_debug_fill_line(
    base: *mut u8,
    pitch: usize,
    max_col: usize,
    bpp_bytes: usize,
    color: u32,
)

Fill a single text line (16 pixel rows) with a solid colour. max_col is the number of character columns (each 8px wide).