Some of the key UI elements you might want to change are the BP_Chat (This is the chat window) the BP_SocialMenu (Main social window) and the BP_GuildSettingsWindow (This is where all the guild permissions are set). There are other widget like the BP_CreateGuildWindow as an example and those can all be found in the Widgets folder and edited in the same way the widget below can be.
The main HUD layout is in the BP_ChatGroupsGuildsHUD here you can change the location of the widgets on screen like the chat, group member panel and self panel. To move the widgets just select them and drag them.
You will need a basic understanding of UE4s widgets to edit these, But below you can see some of the places you change things like colors.
Below are some of the key widgets that control the look of the BP_Chat.
Below are some of the key widget that control the look of the BP_SocialMenu.
Below are some of the key widget that control the look of the BP_GuildSettingsWindow.