config: add [colors-dark] and [colors-light], replacing [colors] and [colors2]

The main reason for having two color sections is to be able to switch
between dark and light. Thus, it's better if the section names reflect
this, rather than the more generic 'colors' and 'colors2' (which was
the dark one and which was the light one, now again?)

When the second color section was added, we kept the original name,
colors, to make sure we didn't break existing configurations, and
third-party themes.

However, in the long run, it's probably better to be specific in the
section naming, to avoid confusion.

So, add 'colors-dark', and 'colors-light'. Keep 'colors' and 'colors2'
as aliases for now, but mark them as deprecated. They WILL be removed
in a future release.

Also rename the option values for initial-color-theme, from 1/2, to
dark/light. Keep the old ones for now, marked as deprecated.

Update all bundled themes to use the new names. In the light-only
themes (i.e. themes that define a single, light, theme), use
colors-light, and set initial-color-theme=light.

Possible improvements: disable color switching if only one color
section has been explicitly configured (todo: figure out how to handle
the default color theme values...)

doc/foot.ini.5.scd

@@ -24,7 +24,7 @@ commented out will usually be installed to */etc/xdg/foot/foot.ini*.
 
 Options are set using KEY=VALUE pairs:
 
-	*\[colors\]*++
+	*\[colors-dark\]*++
 *background=000000*++
 *foreground=ffffff*
 
@@ -371,12 +371,12 @@ empty string to be set, but it must be quoted: *KEY=""*)
 	Default: _yes_
 
 *initial-color-theme*
-	Selects which color theme to use, *1*, or *2*.
+	Selects which color theme to use, *dark*, or *light*.
 	
-	*1* uses the colors defined in the *colors* section, while *2*
-	uses the colors from the *colors2* section.
+	*dark* uses the colors defined in the *colors-dark* section, while
+	*light* uses the colors from the *colors-light* section.
 	
-	Use the *color-theme-switch-1*, *color-theme-switch-2* and
+	Use the *color-theme-switch-dark*, *color-theme-switch-light* and
 	*color-theme-toggle* key bindings to switch between the two themes
 	at runtime, or send SIGUSR1/SIGUSR2 to the foot process (see
 	*foot*(1) for details).
@@ -987,19 +987,24 @@ applications can change these at runtime.
 
 	Default: _400_.
 
-# SECTION: colors
+# SECTION: colors-dark, colors-light
 
-This section controls the 16 ANSI colors, the default foreground and
-background colors, and the extended 256 color palette. Note that
+These two sections controls the 16 ANSI colors, the default foreground
+and background colors, and the extended 256 color palette. Note that
 applications can change these at runtime.
 
 The colors are in RRGGBB format (i.e. plain old 6-digit hex values,
 without prefix). That is, they do *not* have an alpha component. You
 can configure the background transparency with the _alpha_ option.
 
-In the context of private mode 2031 (Dark and Light Mode detection),
-the primary theme (i.e. the *colors* section) is considered to be the
-dark theme (since the default theme is dark).
+*colors-dark* is intended to define a dark color theme, and
+*colors-light* is intended to define a light color theme. You can
+switch between them using the *color-theme-switch-dark*,
+*color-theme-switch-light* and *color-theme-toggle* key bindings, or
+by sending SIGUSR1/SIGUSR2 to the foot process.
+
+The default theme used is *colors-dark*, unless
+*initial-color-theme=light* has been set.
 
 *cursor*
 	Two space separated RRGGBB values (i.e. plain old 6-digit hex
@@ -1098,7 +1103,7 @@ dark theme (since the default theme is dark).
 	black makes the text darker, while blending towards white makes it
 	whiter (but still dimmer than normal text).
 	
-	Default: _black_ (*colors*), _white_ (*colors2*)
+	Default: _black_ (*colors-dark*), _white_ (*colors-light*)
 
 *selection-foreground*, *selection-background*
 	Foreground (text) and background color to use in selected
@@ -1135,20 +1140,6 @@ dark theme (since the default theme is dark).
 	Flash translucency. A value in the range 0.0-1.0, where 0.0 means
 	completely transparent, and 1.0 is opaque. Default: _0.5_.
 
-# SECTION: colors2
-
-This section defines an alternative color theme. It has the exact same
-keys as the *colors* section. The default values are the same, except
-for *dim-blend-towards*, which defaults to *white* instead.
-
-Note that values are not inherited. That is, if you set a value in
-*colors*, but not in *colors2*, the value from *colors* is not
-inherited by *colors2*.
-
-In the context of private mode 2031 (Dark and Light Mode detection),
-the alternative theme (i.e. the *colors2* section) is considered to be
-the light theme (since the default, the primary theme, is dark).
-
 # SECTION: csd
 
 This section controls the look of the _CSDs_ (Client Side
@@ -1455,16 +1446,16 @@ e.g. *search-start=none*.
 	
 	Default: _Control+Shift+u_.
 
-*color-theme-switch-1*, *color-theme-switch-2*, *color-theme-toggle*
-	Switch between the primary color theme (defined in the *colors*
-	section), and the alternative color theme (defined in the
-	*colors2* section).
+*color-theme-switch-dark*, *color-theme-switch-dark*, *color-theme-toggle*
+	Switch between the dark color theme (defined in the *colors-dark*
+	section), and the light color theme (defined in the *colors-light*
+	section).
 	
-	*color-theme-switch-1* applies the primary color theme regardless
+	*color-theme-switch-dark* applies the dark color theme regardless
 	of which color theme is currently active.
 	
-	*color-theme-switch-2* applies the alternative color theme regardless
-	of which color theme is currently active.
+	*color-theme-switch-light* applies the light color theme
+	regardless of which color theme is currently active.
 	
 	*color-theme-toggle* toggles between the primary and alternative
 	color themes.