Add Primer Dark theme

Adds the Primer Dark theme based on the GitHub/Primer colors.

novelwriter/assets/themes/primer_dark.conf

@@ -0,0 +1,71 @@
+[Main]
+name        = Primer Dark
+mode        = dark
+author      = Myian (adaptation)
+credit      = GitHub Inc.
+url         = https://github.com/primer
+license     = MIT License
+licenseurl  = https://github.com/primer/brand/blob/main/LICENSE
+
+[Base]
+default         = #f0f6fc
+faded           = #a3b0be
+red             = #ff7b72
+orange          = #ffa657
+yellow          = #f8e3a1
+green           = #7ee787
+cyan            = #57CCC5
+blue            = #79c0ff
+purple          = #bc8cff
+
+[Project]
+root            = purple
+folder          = orange
+file            = default
+title           = green
+chapter         = red
+scene           = blue
+note            = yellow
+
+[Palette]
+window          = #141b23
+windowtext      = default
+base            = #0c1117
+alternatebase   = #1e242a
+text            = default
+tooltipbase     = yellow
+tooltiptext     = #0c1117
+button          = #212830
+buttontext      = default
+brighttext      = default
+highlight       = #104C35
+highlightedtext = default
+link            = blue
+linkvisited     = blue
+
+[GUI]
+helptext        = faded
+fadedtext       = faded:128
+errortext       = #f85149
+
+[Syntax]
+background     = #0c1117
+text           = faded
+link           = blue
+headertext     = default
+headertag      = faded
+emphasis       = #97ee9e
+dialog         = default
+altdialog      = default
+note           = yellow
+hidden         = faded:128
+shortcode      = #ff9bce
+keyword        = #4493f8
+tag            = blue
+value          = #ffc680
+optional       = #a5d6ff
+spellcheckline = #f85149
+errorline      = cyan
+replacetag     = purple
+modifier       = orange
+texthighlight  = faded:32