List of GUIStyle: http://docs.unity3d.com/ScriptReference/GUIStyle.html

Unity-GUIStyles-list.md

How to Use?

GUIStyle mystyle = new GUIStyle("some string from the list below");

---

UnityEditor.ConsoleWindow.Constants

• "CN Box"
• "Button"
• "ToolbarButton"
• "ToolbarButton"
• "ToolbarButton"
• "ToolbarButton"
• "Toolbar"
• "CN EntryInfo"
• "CN EntryWarn"
• "CN EntryError"
• "CN EntryBackEven"
• "CN EntryBackodd"
• "CN Message"
• "CN StatusError"
• "CN StatusWarn"
• "CN StatusInfo"
• "CN CountBadge"
• "Box"
• "Button"
• "MiniButton"
• "MiniButtonLeft"
• "MiniButtonMiddle"
• "MiniButtonRight"
• "LogStyle"
• "WarningStyle"
• "ErrorStyle"
• "EvenBackground"
• "OddBackground"
• "MessageStyle"
• "StatusError"
• "StatusWarn"
• "StatusLog"
• "Toolbar"
• "CountBadge"

UnityEditor.EditorStyles

• "ColorPickerBox"
• "In BigTitle"
• "miniLabel"
• "LargeLabel"
• "BoldLabel"
• "MiniBoldLabel"
• "WordWrappedLabel"
• "WordWrappedMiniLabel"
• "WhiteLabel"
• "WhiteMiniLabel"
• "WhiteLargeLabel"
• "WhiteBoldLabel"
• "MiniTextField"
• "Radio"
• "miniButton"
• "miniButtonLeft"
• "miniButtonMid"
• "miniButtonRight"
• "toolbar"
• "toolbarbutton"
• "toolbarPopup"
• "toolbarDropDown"
• "toolbarTextField"
• "ToolbarSeachTextField"
• "ToolbarSeachTextFieldPopup"
• "ToolbarSeachCancelButton"
• "ToolbarSeachCancelButtonEmpty"
• "SearchTextField"
• "SearchCancelButton"
• "SearchCancelButtonEmpty"
• "HelpBox"
• "AssetLabel"
• "AssetLabel Partial"
• "AssetLabel Icon"
• "selectionRect"
• "MinMaxHorizontalSliderThumb"
• "DropDownButton"
• "BoldLabel"
• "Label"
• "MiniLabel"
• "MiniBoldLabel"
• "ProgressBarBack"
• "ProgressBarBar"
• "ProgressBarText"
• "FoldoutPreDrop"
• "IN Title"
• "IN TitleText"
• "BoldToggle"
• "Tooltip"
• "NotificationText"
• "NotificationBackground"
• "MiniPopup"
• "textField"
• "ControlLabel"
• "ObjectField"
• "ObjectFieldThumb"
• "ObjectFieldMiniThumb"
• "Toggle"
• "ToggleMixed"
• "ColorField"
• "Foldout"
• "TextFieldDropDown"
• "TextFieldDropDownText"

UnityEditor.ObjectListArea.Styles

• "PR Label"
• "ProjectBrowserGridLabel"
• "ObjectPickerResultsGrid"
• "ObjectPickerBackground"
• "ObjectPickerPreviewBackground"
• "ProjectBrowserHeaderBgMiddle"
• "ProjectBrowserHeaderBgTop"
• "Label"
• "MiniLabel"
• "Foldout"
• "ObjectPickerToolbar"
• "PR TextField"
• "PR Ping"
• "PR Ping"
• "ProjectBrowserIconDropShadow"
• "ProjectBrowserTextureIconDropShadow"
• "ProjectBrowserIconAreaBg"
• "ProjectBrowserPreviewBg"
• "ProjectBrowserSubAssetBg"
• "ProjectBrowserSubAssetBgOpenEnded"
• "ProjectBrowserSubAssetBgCloseEnded"
• "ProjectBrowserSubAssetBgMiddle"
• "ProjectBrowserSubAssetBgDivider"
• "ProjectBrowserSubAssetExpandBtn"

Or you simply checkout the Unity source code ;)
e.g. https://github.com/Unity-Technologies/UnityCsReference/blob/master/Editor/Mono/GUI/EditorStyles.cs