Improve themes handling

- Store user theme in session
- Logged out users will keep their theme
- Add theme background color to web manifest and meta tag

template/functions.go

@@ -15,6 +15,7 @@ import (
 	"github.com/miniflux/miniflux/config"
 	"github.com/miniflux/miniflux/filter"
 	"github.com/miniflux/miniflux/http/route"
+	"github.com/miniflux/miniflux/model"
 	"github.com/miniflux/miniflux/url"
 )
 
@@ -90,6 +91,9 @@ func (f *funcMap) Map() template.FuncMap {
 
 			return str
 		},
+		"theme_color": func(theme string) string {
+			return model.ThemeColor(theme)
+		},
 
 		// These functions are overrided at runtime after the parsing.
 		"elapsed": func(timezone string, t time.Time) string {