How do I add horizontal grid in Forestplot

[DelGo]

Hi all, this is the first time I am making a forest plot. I have some questions. I hope some of you can help.

1. I am trying to add horizontal grid to a forest plot as a guide to read the OR and its 95% CI provided on the right. How do I do this?
2. Is there a way I can format zero line (line type and line width)?
3. Can I add an additional vertical line at 0?

The code is as follows:

step_cluster_obese <- 
  structure(list(
    OR  = c(NA, NA, 0.38, 1.09, 1.41, 2.05, 2.05, 2.41, 3.15), 
    lower = c(NA, NA, 0.15,	0.50,	0.82,	1.15,	1.30, 1.39,	1.69),
    upper = c(NA, NA, 0.93,	2.38,	2.40,	3.63,	3.21,	4.19,	5.87)),
    .Names = c("OR", "lower", "upper"), 
    row.names = c(NA, -11L), 
    class = "data.frame")

tabletext<-cbind(
  c("", "Factor", "a", "b", "c", "d", "e", "f", "g"),
  c("", "OR (95% CI)", "0.38 (0.15, 0.93)", "1.09 (0.50, 2.38)", "1.41 (0.82, 2.40)", "2.05 (1.15, 3.63)", "2.05 (1.30, 3.21)", "2.41 (1.39, 4.19)", "3.15 (1.69, 5.87)"))

forestplot(tabletext, 
           graph.pos = 2, step_cluster_obese, new_page = TRUE,  boxsize = .15, 
           is.summary=c(TRUE,TRUE,rep(FALSE,8),TRUE),
           clip=c(0.1,6.5), 
           xlog=FALSE,
           zero = 1, 
           col=fpColors(box = "black", lines = "black", summary = "black", zero = "grey", text = "black", axes = "black", hrz_lines = "black"),        
           vertices = TRUE,
           xlab="Odds Ratio",
           xticks = c(0, 1, 2, 3, 4, 5, 6),
           lwd.zero = 0, 
           txt_gp = fpTxtGp(label = list(gpar(fontfamily = "sans", cex = 1.5),
                                         gpar(fontfamily = "")),
                            ticks = gpar(fontfamily = "", cex=1.5),
                            xlab  = gpar(fontfamily = "sans", cex = 1.5)))