Add cex as explicit argument to tinytheme

[grantmcdermott]

Closes #477

pkgload::load_all('~/Documents/Projects/tinyplot')
#> ℹ Loading tinyplot
tinyplot(dist ~ speed, data = cars)

tinytheme("clean2", pch = 17, cex = 3)
tinyplot(dist ~ speed, data = cars)

tinytheme()
tinyplot(dist ~ speed, data = cars)

^{Created on 2025-09-16 with reprex v2.1.1}