[FEATURE] Improve Theme

[rsh52]

Feature Request Description

Improve theme for theme_ggswim().

Checklist

• [x] The issue is atomic
• [x] The issue description is documented
• [x] The issue title describes the problem succinctly
• [x] Developers are assigned to the issue
• [x] Labels are assigned to the issue

[rsh52]

Changes made to theme_ggswim() and new theme_ggswim_dark(), see gallery in pkgdown