From what I read, here :

http://yaml.org/spec/1.2/spec.html#id2760844

Text styling seems to work inside the block notations, beginning with the | or the > symbols

Here is, below, some blocks of random YAML code, ( without any coherence ! ) just to see the color effects of some structures !

Put that text in a new tab and choose the YAML language

#---------------------------------------------
string: '012345'
date: 2002-12-14
boolean: true
%TAG ! tag:clarkevans.com,2002:
--- !shape
  # Use the ! handle for presenting
  # tag:clarkevans.com,2002:circle
- !circle
  center: &ORIGIN {x: 73, y: 129}
  radius: 7
- !line
  start: *ORIGIN
  finish: { x: 89, y: 102 }
- !label
  start: *ORIGIN
  color: 0xFFEEBB
  text: Pretty vector drawing.
literal_block: |
    All line-break
        are significant
folded block: >
    each line break is folded to a space
        unless it ends an empty or a more-indented line. 
#---------------------------------------------
%TAG ! tag:clarkevans.com,2002:
--- !shape
  # Use the ! handle for presenting
  # tag:clarkevans.com,2002:circle
- !circle
  center: &ORIGIN {x: 73, y: 129}
  radius: 7
- !line
  start: *ORIGIN
  finish: { x: 89, y: 102 }
- !label
  start: *ORIGIN
  color: 0xFFEEBB
  text: Pretty vector drawing.
#---------------------------------------------
--- !<tag:clarkevans.com,2002:invoice>
invoice: 34843
date   : 2001-01-23
bill-to: &id001
    given  : Chris
    family : Dumars
    address:
        lines: |
            458 Walkman Dr.
            Suite #292
        city    : Royal Oak
        state   : MI
        postal  : 48046
ship-to: *id001
product:
    - sku         : BL394D
      quantity    : 4
      description : Basketball
      price       : 450.00
    - sku         : BL4438H
      quantity    : 1
      description : Super Hoop
      price       : 2392.00
tax  : 251.42
total: 4443.52
comments:
    Late afternoon is best.
    Backup contact is Nancy
    Billsmer @ 338-4338.
#---------------------------------------------
---
Time: 2001-11-23 15:01:42 -5
User: ed
Warning:
  This is an error message
  for the log file
---
Time: 2001-11-23 15:02:31 -5
User: ed
Warning:
  A slightly different error
  message.
---
Date: 2001-11-23 15:03:17 -5
User: ed
Fatal:
  Unknown variable "bar"
Stack:
  - file: TopClass.py
    line: 23
    code: |
      x = MoreObject("345\n")
  - file: MoreClass.py
    line: 58
    code: |-
      foo = bar
#---------------------------------------------

Best Regards,

guy038

The list of categories shown in the “style” column of the Configurator, and how they are parsed/lexed, are defined by the lexer (in this case, the YAML lexer), which Notepad++ inherits from Scintilla (the underlying editing component that Notepad++ uses). I tried looking through NPP’s copy of the Scintilla YAML lexer source code – but I don’t know enough about lexer code to fully follow, and I’m not immediately seeing how it defines the beginning/end of “TEXT”. It seems to have something to do with whether indenting has changed… but I’m not sure exactly how, and I couldn’t get any pseudo-YAML text I entered to be interpreted as “TEXT”. But it doesn’t seem that “TEXT” means “something in double quotes”. Maybe “STRING” does (but I don’t see it in my Style Configurator, nor in the NPP LexYAML source code, so I don’t know…)

If you wanted to ask for a change in the lexer, such that it had a “STRING” or similar “QUOTED” category, you’d have to go to the Scintilla project, and make an issue there – after verifying that such an issue doesn’t exist, and after verifying that the most recent Scintilla doesn’t already have it. (It’s quite possible it does: Notepad++ doesn’t upgrade its Scintilla version very often – 3.56 appears to be the version NPP uses, since mid-2015.)