Simplify default style and match Rust code to config (#9900)

# Description

This PR aligns the default config in the Rust code to the default config
in the nushell file.

To do so, it removes closures from the default file, opting instead to
pick a simple style as default. This allows easier maintenance of both
Rust and Nushell code without removing the ability to use closures for
styling in your configuration.

The default theme is now "dark mode" in both the Rust and nushell config
code.

Obligatory screenshot:


![image](https://github.com/nushell/nushell/assets/547158/233b11af-3b81-4513-8a69-3e7b1cac3865)


# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->

# Tests + Formatting
<!--
Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect -A clippy::result_large_err` to check that
you're using the standard code style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"` to run the tests for the standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

# After Submitting
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documentation](https://github.com/nushell/nushell.github.io) after the
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crates/nu-color-config/src/style_computer.rs

@@ -143,13 +143,13 @@ impl<'a> StyleComputer<'a> {
         let mut map: StyleMapping = [
             ("separator".to_string(), ComputableStyle::Static(Color::White.normal())),
             ("leading_trailing_space_bg".to_string(), ComputableStyle::Static(Style::default().on(Color::Rgb(128, 128, 128)))),
-            ("header".to_string(), ComputableStyle::Static(Color::White.normal())),
-            ("empty".to_string(), ComputableStyle::Static(Color::White.normal())),
-            ("bool".to_string(), ComputableStyle::Static(Color::White.normal())),
+            ("header".to_string(), ComputableStyle::Static(Color::Green.bold())),
+            ("empty".to_string(), ComputableStyle::Static(Color::Blue.normal())),
+            ("bool".to_string(), ComputableStyle::Static(Color::LightCyan.normal())),
             ("int".to_string(), ComputableStyle::Static(Color::White.normal())),
-            ("filesize".to_string(), ComputableStyle::Static(Color::White.normal())),
+            ("filesize".to_string(), ComputableStyle::Static(Color::Cyan.normal())),
             ("duration".to_string(), ComputableStyle::Static(Color::White.normal())),
-            ("date".to_string(), ComputableStyle::Static(Color::White.normal())),
+            ("date".to_string(), ComputableStyle::Static(Color::Purple.normal())),
             ("range".to_string(), ComputableStyle::Static(Color::White.normal())),
             ("float".to_string(), ComputableStyle::Static(Color::White.normal())),
             ("string".to_string(), ComputableStyle::Static(Color::White.normal())),

crates/nu-utils/src/sample_config/default_config.nu

@@ -14,33 +14,12 @@ let dark_theme = {
     empty: blue
     # Closures can be used to choose colors for specific values.
     # The value (in this case, a bool) is piped into the closure.
-    bool: {|| if $in { 'light_cyan' } else { 'light_gray' } }
+    # eg) {|| if $in { 'light_cyan' } else { 'light_gray' } }
+    bool: light_cyan
     int: white
-    filesize: {|e|
-        if $e == 0b {
-            'white'
-        } else if $e < 1mb {
-            'cyan'
-        } else { 'blue' }
-    }
+    filesize: cyan
     duration: white
-    date: {|| (date now) - $in |
-        if $in < 1hr {
-            'purple'
-        } else if $in < 6hr {
-            'red'
-        } else if $in < 1day {
-            'yellow'
-        } else if $in < 3day {
-            'green'
-        } else if $in < 1wk {
-            'light_green'
-        } else if $in < 6wk {
-            'cyan'
-        } else if $in < 52wk {
-            'blue'
-        } else { 'dark_gray' }
-    }
+    date: purple
     range: white
     float: white
     string: white
@@ -98,33 +77,12 @@ let light_theme = {
     empty: blue
     # Closures can be used to choose colors for specific values.
     # The value (in this case, a bool) is piped into the closure.
-    bool: {|| if $in { 'dark_cyan' } else { 'dark_gray' } }
+    # eg) {|| if $in { 'dark_cyan' } else { 'dark_gray' } }
+    bool: dark_cyan
     int: dark_gray
-    filesize: {|e|
-        if $e == 0b {
-            'dark_gray'
-        } else if $e < 1mb {
-            'cyan_bold'
-        } else { 'blue_bold' }
-    }
+    filesize: cyan_bold
     duration: dark_gray
-    date: {|| (date now) - $in |
-        if $in < 1hr {
-            'purple'
-        } else if $in < 6hr {
-            'red'
-        } else if $in < 1day {
-            'yellow'
-        } else if $in < 3day {
-            'green'
-        } else if $in < 1wk {
-            'light_green'
-        } else if $in < 6wk {
-            'cyan'
-        } else if $in < 52wk {
-            'blue'
-        } else { 'dark_gray' }
-    }
+    date: purple
     range: dark_gray
     float: dark_gray
     string: dark_gray
@@ -275,7 +233,7 @@ $env.config = {
         vi_normal: underscore # block, underscore, line, blink_block, blink_underscore, blink_line (underscore is the default)
     }
 
-    color_config: {} # if you want a more interesting theme, you can replace the empty record with `$dark_theme`, `$light_theme` or another custom record
+    color_config: $dark_theme # if you want a more interesting theme, you can replace the empty record with `$dark_theme`, `$light_theme` or another custom record
     use_grid_icons: true
     footer_mode: "25" # always, never, number_of_rows, auto
     float_precision: 2 # the precision for displaying floats in tables