feature: maybe set DiagnosticOk highlights

[fredrikfoss]

Describe the solution you'd like

Hey, this is minor but I by chance saw that the DiagnosticOk highlight isn't set by the colorscheme. Right now it uses guifg=NvimLightGreen. I don't mind the color, but the DiagnosticVirtualTextOk highlight won't get a blended background like the other 4 ones :)

Additional context

$ nvim -v
NVIM v0.10.0-dev+2663-gc1c6c1ee1
Build type: RelWithDebInfo
LuaJIT 2.1.1707061634
Run "nvim -V1 -v" for more info

[mvllow]

Do you have a screenshot/example of when this highlight is used? Happy to add support here but not sure how to test