Add theme variants

- Use CSS variables instead of inherence
- Rename default theme to "Light - Serif"
- Rename Black theme to "Dark - Serif"
- Rename "Sans-Serif" theme to "Light - Sans Serif"
- Add "System" theme that use system preferences: Dark or Light
- Add Serif and Sans-Serif variants for each color theme

model/theme_test.go

@@ -7,7 +7,7 @@ package model // import "miniflux.app/model"
 import "testing"
 
 func TestValidateTheme(t *testing.T) {
-	for _, status := range []string{"default", "black", "sansserif"} {
+	for _, status := range []string{"light_serif", "dark_sans_serif", "system_serif"} {
 		if err := ValidateTheme(status); err != nil {
 			t.Error(`A valid theme should not generate any error`)
 		}