change Dark theme to be actually dark; add theme docs

docs/md/Customize-theme-colors.md

@@ -0,0 +1,36 @@
+# Customize theme (colors)
+
+**Available in version 3.6 or later.**
+
+You can change colors of SumatraPDF UI by creating a custom theme using `Themes` [advanced setting](Advanced-options-settings.md).
+
+To create a theme:
+
+- use `Settings` / `Advanced Options...` menu (or `Ctrl + K` Command Palette, type `adv` to narrow down and select `Advanced Options...` command)
+- this opens default .txt editor with advanced settings file
+- find `Themes` array and add new shortcut definitions
+
+Example of customization:
+
+```
+Shortcuts [
+    [
+        Name = My Dark Theme
+        TextColor = #bac9d0
+        BackgroundColor = #263238
+        ControlBackgroundColor = #263238
+        LinkColor = #8aa3b0
+        ColorizeControls = true
+    ]
+]
+```
+
+`TextColor` and `BackgroundColor` are for main window color and color of text.
+
+If you use `I` (`CmdInvertColors`) they will also be used to replace white background / black text color when renderign PDF/ePub documents.
+
+`ControlBackgroundColor` is for background of Windows controls (buttons, window frame, menus, list controls etc.).
+
+`LinkColor` is a color for links. Typically it's blue.
+
+`ColorizeControls` should be `true`. If `false` we won't try to change colors of standard windows controls (menu, toolbar, buttons etc.) so a lot of UI will not respect theme colors.

docs/md/SumatraPDF-documentation.md

@@ -7,18 +7,20 @@
 
 [Keyboard shortcuts](Keyboard-shortcuts.md)
 
-[Scrolling, zooming, navigating](Scrolling-and-zooming.md)
-
 [Customizing keyboard shortcuts](Customizing-keyboard-shortcuts.md)
 
-[Customizing toolbar](Customize-toolbar.md)
+[Scrolling, zooming, navigating](Scrolling-and-zooming.md)
 
 [Commands](Commands.md)
 
 [Command-line arguments](Command-line-arguments.md)
 
 [Advanced options / settings](Advanced-options-settings.md)
 
+[Customizing toolbar](Customize-toolbar.md)
+
+[Customizing UI colors / theme](Customize-theme-colors.md)
+
 [Customize external viewers](Customize-external-viewers.md)
 
 [Customize search / translation services](Customize-search-translation-services.md)

src/Theme.cpp

@@ -36,6 +36,14 @@ static const char* themesTxt = R"(Themes [
         LinkColor = #8aa3b0
         ColorizeControls = true
     ]
+    [
+        Name = Dark from 3.5
+        TextColor = #bac9d0
+        BackgroundColor = #263238
+        ControlBackgroundColor = #263238
+        LinkColor = #8aa3b0
+        ColorizeControls = true
+    ]
     [
         Name = Darker
         TextColor = #c3c3c6