I created fasm syntax highlighting style for the current version of Notepad ++ (NPP_v7.7.1, UDL2.1).
The style is defined only for the lower case, but this is a matter of taste, and is solved by setting one checkmark in the syntax settings.
Defined folds for macros and conditional assembly directives. The list of x86-64 instructions was manually pulled from the latest version of Intel Software Developer Manuals.
Among the shortcomings: I could not get notepad ++ to highlight hexadecimal numbers with the suffix 'h' with hexadecimal digits in lower case so that it would not break the highlighting of binary numbers with the suffix 'b' and floating-point numbers with the suffix 'f'. Therefore, hexadecimal numbers with the suffix 'h' are correctly highlighted only if they use hexadecimal digits in upper case. Hexadecimal numbers with prefixes are determined correctly, regardless of the case of digits.

You could also use fasm's TABLES.INC file to get the opcodes, directives and operators.

I would recommend that, for several reasons:

• Some instruction sets like AMD 3DNow! never were in any Intel manuals.
  
• Some instructions, like FNENI/FNDISI, were doing something only on old generations of chips and are no longer listed in current manuals (but fasm still knows them).
  
• SALC remains undocumented, even though all CPUs seem to support it (also fasm allows alternate name SETALC - as an undocumented instruction it was sometimes known under different names).

Thank you for your advice!
I will try to follow it as soon as free time appears.

The style has lead in accordance with the file "TABLES.INC" from the fasm_v1.73.16 source code.
Changed highlight colors.
Added auto-highlighting of files with the extensions ".asm" and ".inc".

Hi, iVeryBadGuy!
Is your second nick Shere Khan?

Hi, Mikl___!
Not in this forum.
My nick is Shere Khan on wasm.in.

The new version of the highlight style, from 2019-12-03.

Changes:
The style has lead in accordance with the file "TABLES.INC" from the fasm_v1.73.20 source code.

Thank you.

You'll most likely want to:

Settings -> Style Configurator... "Enable global background color".