[feature] User-selectable preset CSS themes for accounts (#2777)

* [feature] User-selectable preset themes

* docs, more theme stuff

* lint, tests

* fix css name

* correct some little issues

* add another theme

* fix poll background

* okay last theme i swear

* make retrieval of apimodel themes more conventional

* preallocate stylesheet slices

internal/api/client/accounts/accountupdate.go

@@ -309,6 +309,7 @@ func parseUpdateAccountForm(c *gin.Context) (*apimodel.UpdateCredentialsRequest,
 			form.Source.Language == nil &&
 			form.Source.StatusContentType == nil &&
 			form.FieldsAttributes == nil &&
+			form.Theme == nil &&
 			form.CustomCSS == nil &&
 			form.EnableRSS == nil) {
 		return nil, errors.New("empty form submitted")