My lookup page for all Standard and Font APEX icons - and also Font Awesome, for older Apex version that include that library.
You do need to grant select on APEX_240100.wwv_flow_standard_icons to your schema for this to work.

More info on Font Awesome and using it alongside Font APEX can by found on this page: Font Awesome. That page also explains the use of CSS class fawe instead of fa.

The official Apex Theme 42 icons page only shows Font APEX, it can be found here: https://apex.oracle.com/pls/apex/f?p=42:icons.

Page

Identification
	Page ID	1003
	Name	Apex - Images & Fonts - Standard Icons & Fonts

View

View
	Name	TED_P1003_V
	DDL	with a as ( select v('P1003_MAX_IMAGE_SIZE') as max_image_size , nv('P1003_FONT_SIZE') as font_size from dual ) , b as ( select i.id , i.icon_library , ( case i.icon_library when 'FONTAWESOME' then 'fawe' || substr(i.icon_name, 3) else i.icon_name end ) as icon_name , i.icon_size , lower( replace(i.icon_category,':',': ') ) as icon_category , replace(i.icon_filters, ',', ', ') as icon_filters , ( case i.icon_library when 'APEX' then '<img src="/i/menu/' || i.icon_name || '" style="max-width:' || a.max_image_size || 'px;" alt="" title="" />' when 'FONTAWESOME' then '<span aria-hidden="true" class="fawe ' || ( case when a.font_size > 1 then 'fawe-' || trim(a.font_size) || 'x ' else null end ) || 'fawe' || substr(i.icon_name, 3) || '"></span>' when 'FONTAPEX' then '<span aria-hidden="true" class="fa ' || ( case when a.font_size > 1 then 'fa-' || trim(a.font_size) || 'x ' else null end ) || i.icon_name || '"></span>' else null end ) as preview from a , apex_240100.wwv_flow_standard_icons i ) select b."ID",b."ICON_LIBRARY",b."ICON_NAME",b."ICON_SIZE",b."ICON_CATEGORY",b."ICON_FILTERS",b."PREVIEW" , replace(b.preview, '/i/', '#IMAGE_PREFIX#') as html from b

Item

Identification
	Sequence	10
	Name	P1003_MAX_IMAGE_SIZE
	Display As	Select List
User Interface
	Region	Standard Icons & Fonts
List Of Values
	Display Extra Values	Yes
	Display Null Value	No
	List of values definition	STATIC2:16;16,32;32,64;64,128;128,max;9999
Source
	Source Used	Only when current value in session state is null
	Source Type	Static Assignment (value equals source attribute)
	Source value or expression	32

Item

Identification
	Sequence	20
	Name	P1003_FONT_SIZE
	Display As	Select List
User Interface
	Region	Standard Icons & Fonts
List Of Values
	Display Extra Values	Yes
	Display Null Value	No
	List of values definition	STATIC2:1x;1,2x;2,3x;3,4x;4,5x;5
Source
	Source Used	Only when current value in session state is null
	Source Type	Static Assignment (value equals source attribute)
	Source value or expression	2