Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="theme-1">
  <div class="side-nav">This is theme 1</div>
</div>
<div class="theme-2">
  <div class="side-nav">This is theme 2</div>
</div>

SassMeister-input.scss

// ----
// Sass (v3.4.20)
// Compass (v1.0.3)
// ----

/** context:
I wanted to set up a Sass map for my colour palette. I am nearly there, but
I want to set up validation so that if a non-existant palette or tone is specified in
the Sass files, the developer gets a warning 
**/

/** issue:
As it stands, if I specify a non-existant palette e.g. theme-3 which doesn't exist, the
error in the terminal is "$map: null is not a map for `map-get'". However, if I specify
a non-existant tone e.g. 50pc which doesn't exist, there are no errors - the property
is just not printed in the compiled CSS.
**/

/** desired solution:
I would like error messages for both invalid palette and tone.
**/

$theme-color-1: #ff6600;
$theme-color-2: #000000;

@function color-palette($palette, $tone: 'base') {
	@if (map-has-key($color-palette, $palette), $tone) {
		@if $palette {
			@return map-get(map-get($color-palette, $palette), $tone);
			@if $tone {
				@return map-get(map-get($color-palette, $palette), $tone);
			} @else {
				@warn "Unknown `#{$tone}` in $color-palette.";
  			@return null;
			}
		} @else {
			@warn "Unknown `#{$palette}` in $color-palette.";
			@return null;
		}
	}
}

$color-palette: (
	theme-1: (
		base: $theme-color-1,
		10pc: rgba($theme-color-1, 0.1),
		25pc: rgba($theme-color-1, 0.25)
	),
	theme-2: (
		base: $theme-color-2,
		10pc: rgba($theme-color-2, 0.1),
		25pc: rgba($theme-color-2, 0.25)
	),
);

.theme-1 {
	.side-nav {
		background-color: color-palette(theme-1, 10pc);
		color: color-palette(theme-1, 25pc);

	}
}

.theme-2 {
	.side-nav {
		background-color: color-palette(theme-2, 10pc);
		color: color-palette(theme-2, 25pc);
	}
}

SassMeister-output.css

/** context:
I wanted to set up a Sass map for my colour palette. I am nearly there, but
I want to set up validation so that if a non-existant palette or tone is specified in
the Sass files, the developer gets a warning 
**/
/** issue:
As it stands, if I specify a non-existant palette e.g. theme-3 which doesn't exist, the
error in the terminal is "$map: null is not a map for `map-get'". However, if I specify
a non-existant tone e.g. 50pc which doesn't exist, there are no errors - the property
is just not printed in the compiled CSS.
**/
/** desired solution:
I would like error messages for both invalid palette and tone.
**/
.theme-1 .side-nav {
  background-color: rgba(255, 102, 0, 0.1);
  color: rgba(255, 102, 0, 0.25);
}

.theme-2 .side-nav {
  background-color: rgba(0, 0, 0, 0.1);
  color: rgba(0, 0, 0, 0.25);
}

SassMeister-rendered.html

<div class="theme-1">
  <div class="side-nav">This is theme 1</div>
</div>
<div class="theme-2">
  <div class="side-nav">This is theme 2</div>
</div>