Incorporate ink, paper theme settings

[teunbrand]

This PR doesn't correspond to an issue, and is more like upkeep.

Briefly, all geoms now listen to the theme(geom) field and also theme_graph() and set_graph_style() work with ink/paper.

library(tidygraph)
#> 
#> Attaching package: 'tidygraph'
#> The following object is masked from 'package:stats':
#> 
#>     filter
devtools::load_all("~/packages/test/ggraph/")
#> ℹ Loading ggraph
#> Loading required package: ggplot2

graph <- as_tbl_graph(highschool) |> 
  mutate(Popularity = centrality_degree(mode = 'in'))

ggraph(graph, layout = 'kk') + 
  geom_edge_fan(aes(alpha = after_stat(index)), show.legend = FALSE) + 
  geom_node_point(aes(size = Popularity)) + 
  facet_edges(~year) + 
  theme_graph(ink = "red", paper = "cornsilk")

^{Created on 2025-10-03 with reprex v2.1.1}