Add support for themes (fixes #1925)

lib/config/config.go

@@ -48,6 +48,9 @@ var (
 	// DefaultDiscoveryServers should be substituted when the configuration
 	// contains <globalAnnounceServer>default</globalAnnounceServer>.
 	DefaultDiscoveryServers = append(DefaultDiscoveryServersV4, DefaultDiscoveryServersV6...)
+
+	// DefaultTheme is the default and fallback theme for the web UI.
+	DefaultTheme = "default"
 )
 
 func New(myID protocol.DeviceID) Configuration {

lib/config/guiconfiguration.go

@@ -20,6 +20,7 @@ type GUIConfiguration struct {
 	RawUseTLS           bool   `xml:"tls,attr" json:"useTLS"`
 	RawAPIKey           string `xml:"apikey,omitempty" json:"apiKey"`
 	InsecureAdminAccess bool   `xml:"insecureAdminAccess,omitempty" json:"insecureAdminAccess"`
+	Theme               string `xml:"theme" json:"theme" default:"default"`
 }
 
 func (c GUIConfiguration) Address() string {