How far off is the default ggsurvplot output from the rules you have to comply with? Have you looked into the customization options survminer already has? Colors, font sizes, etc can all be changed with arguments to ggsurvplot() or to the underlying theme_survminer() and theme_cleantable(). See:
http://www.sthda.com/english/rpkgs/survminer/reference/ggsurvtheme.html
http://www.sthda.com/english/rpkgs/survminer/reference/ggsurvplot.html
But the theming is also compatible with the usual ggplot2 methods, so there's not very many limits to what you can do. Some silly examples...
library("survival")
library("survminer")
#> Loading required package: ggplot2
#> Loading required package: ggpubr
#> Loading required package: magrittr
library("ggthemes")
fit<- survfit(Surv(time, status) ~ sex, data = lung)
# Hideous vintage Excel survival plot!
ggsurvplot(
fit, data = lung, risk.table = TRUE,
palette = ggthemes::excel_pal()(2),
ggtheme = ggthemes::theme_excel(),
tables.theme = survminer::theme_cleantable() +
theme(panel.grid = element_blank())
)

# Slightly less ridiculous example
ggsurvplot(
fit, data = lung, risk.table = TRUE,
palette = "lancet",
ggtheme = ggthemes::theme_stata() + theme(legend.title = element_text(size = 12, vjust = 0.5)),
tables.theme = survminer::theme_cleantable() +
theme(panel.grid = element_blank(),
axis.text.y = element_text(angle = 0)
)
)

Created on 2018-10-18 by the reprex package (v0.2.1)