create minimal dxfs styles

[JanMarvin]

See ycphs/openxlsx#387 (comment)

> old <- create_dxfs_style(font_color = wb_colour(name = "black"), bgFill = wb_colour(hex = "FFBFBFBF"))
> new <- "<dxf><font/><fill><patternFill patternType=\"solid\"><bgColor rgb=\"FFBFBFBF\"/></patternFill></fill></dxf>"
> 
> old %>% as_xml()

<dxf>
 <font>
  <color rgb="FF000000" />
  <name val="Calibri" />
  <sz val="11" />
 </font>
 <fill>
  <patternFill patternType="solid">
   <bgColor rgb="FFBFBFBF" />
  </patternFill>
 </fill>
</dxf>

> new %>% as_xml()

<dxf>
 <font />
 <fill>
  <patternFill patternType="solid">
   <bgColor rgb="FFBFBFBF" />
  </patternFill>
 </fill>
</dxf>

Workaround

library(openxlsx2)

wb <- wb_workbook()$
  add_worksheet("Tab_1", zoom = 80, gridLines = FALSE)$
  add_worksheet("Tab_2", zoom = 80, gridLines = FALSE)

# ccs_gray <- create_dxfs_style(font_color = wb_colour(name = "black"), bgFill = wb_colour(hex = "FFBFBFBF"))
ccs_gray <- "<dxf><font/><fill><patternFill patternType=\"solid\"><bgColor rgb=\"FFBFBFBF\"/></patternFill></fill></dxf>"
wb$styles_mgr$add(ccs_gray, "ccs_gray")

wb <- wb_add_conditional_formatting(
  wb = wb,
  sheet = "Tab_1",
  cols = 1:5,
  rows = 1:10,
  rule = "MOD(ROW(A1), 2) = 0",
  style = "ccs_gray",
  type = "expression")

hl <- function(row = i) create_hyperlink(sheet = "Tab_2", row = i, col = 1, text = paste0("Link to 'Tab_2'"))

for(i in 1:10) {
  wb <- wb_add_formula(wb = wb, sheet = "Tab_1", startRow = i, startCol = 1, x = hl(i))
  for(j in 2:5) {
    wb <- wb_add_data(wb = wb, sheet = "Tab_1", x = "Some text", startRow = i, startCol = j)
  }
}

wb$open()