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I find the default highlighting for selecting text far too pale -- often I am not sure how much of the text I have highlighted. Similarly, I like coloured strike-through to make the effect more visible. How do I change the settings so as to make my preferred effect the default setting ?

The text selection color in Nisus Writer can indeed be customized:

1. Open your Nisus Writer preferences.
2. Switch to the Appearance pane.
3. Look at the bottom of the pane for the label "Colors".
4. Choose the "Selection" category, inside which you'll see a host of colors you can customize.

The color you're talking about is the "Active selection" color, but you can set colors for a variety of circumstances, eg: find & replace matches, text insertion point, etc.

Hope that helps!

I find the default highlighting for selecting text far too pale

To change what Nisus calls "Active selection", go to Preferences > Colors > Selection > Active selection > Active selection color

I like coloured strike-through to make the effect more visible. How do I change the settings

You could create a character style with your preferred settings, or use a macro for this. In both cases you would want to use a keyboard shortcut because this is an action you will probably frequently perform.
You seem to be new to Nisus, so I will give a detailed step-by-step instruction below.

The first option (character style):
1. Select some text
2. Format > Strikethrough > Single. The selected text has now default black strikethrough line
3. Choose Format > Strikethrough > Show Strikethrough Colors
4. Apply your color to the selection
5. Now create a new Character Style based on the selection: Format > Character Style > New Character Style Based on Selection
6. Assign a keyboard shortcut to the new character style, either via the shortcut field in the Style Sheet or Preferences > Menu Keys > Format > Character Style > [name of your new character style]

The second option
(1) Install one or both of the following macros.
(2) Assign them keyboard shortcuts via Preferences > Menu Keys. 

I use Com d for "strikethrough", Com dr for "strikethrough with red", and Com db for "strikethrough with blue."

---------------------------------------------
# Strikethrough with RED color
$red = Color.red
Set Strikethrough 1
# 1 = single line strike, 2 = double line strike, 3 = thick, single line strike, 4 = thick, double line strike, 5 = dotted line strike

Set Strikethrough Color $red
-----------------------------------------------
# Strikethrough with BLUE color

$blue = Color.blue
Set Strikethrough 1
Set Strikethrough Color $blue

Thank you ! The selection-highlighting is just what I wanted, and I shall investigatethe proposed formula for strike-through when I have got a little more time !