Quick: Support Variable in x-truncate CSS

[wesleyboar]

Overview

Support a custom property (variable) and a fallback value (the original value) for the value of a rule in the x-truncate "tool" stylesheet.

Changes

• Swap static value for dynamic value with fallback.

Testing / Screenshots

Ignoring, because:

• This small change was successfully tested during development of #75 (still in progress as of 2021-08-03).
• This small change is already proven to be reliable via some merged Core-Portal PR by me I have trouble finding.

Notes

• This feature is documented by MDN: Using CSS custom properties @ Custom property fallback values.
• This is needed for "Data List" Table (for System Specs) - Styles & Plugin #75 and I try to reduce the size of that PR.

[wesleyboar]

I self-merge for the reasons listed in "Testing / Screenshots" and "Notes" in description.

quoted content

Testing / Screenshots

Ignoring, because:

• This small change was successfully tested during development of #75 (still in progress as of 2021-08-03).
• This small change is already proven to be reliable via some merged Core-Portal PR by me I have trouble finding.

Notes

• This feature is documented by MDN: Using CSS custom properties @ Custom property fallback values.
• This is needed for "Data List" Table (for System Specs) - Styles & Plugin #75 and I try to reduce the size of that PR.