GeezerVison

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

• argbFromColorMix() returns -1 when @csstools/css-color-parser cannot parse the value, and ColorBase stores it as the color. That is opaque white on Android, and transparent blue (invisible) on iOS, which builds the color from the r/g/b/a getters.
• The usual trigger is an unresolved variable nested in a function. _evaluateCssVariableExpression substitutes unset inline, so color-mix(in srgb, var(--missing) 35%, transparent) becomes color-mix(in srgb, unset 35%, transparent), which fails to parse.
• The same inline substitution makes calc(var(--missing) + 10) evaluate to 10.

What is the new behavior?

This follows the CSS spec:

• Unresolved var(): a var() with no value (undefined, empty, or unset) and no fallback that resolves makes the whole declaration invalid at computed-value time, so the property is unset. This holds wherever the var() is nested: color-mix(), calc(), shorthands, multi-value strings. An unresolved var() inside the fallback of a var() that did resolve is ignored, as in browsers: var(--defined, var(--missing)) gives --defined.
• Malformed color-mix(): throws Invalid color: <value>, like every other unparseable color string. The cascade already catches and traces that, so the declaration is dropped instead of painting white.

Behavior changes:

• calc(var(--missing) + 10) is now unset instead of 10.
• margin: var(--a) var(--missing) now unsets the whole shorthand instead of only the missing part.
• The color-mix(in --swop5c, red, blue) spec used to assert the decoded -1 sentinel. It is now a throw case: without a color profile the value cannot be parsed, and browsers drop it too.