[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/processing/account/account.go

@@ -44,6 +44,7 @@ type Processor struct {
 	formatter    *text.Formatter
 	federator    *federation.Federator
 	parseMention gtsmodel.ParseMentionFunc
+	themes       *Themes
 }
 
 // New returns a new account processor.
@@ -67,5 +68,6 @@ func New(
 		formatter:    text.NewFormatter(state.DB),
 		federator:    federator,
 		parseMention: parseMention,
+		themes:       PopulateThemes(),
 	}
 }