tools: check-theme: consider background-color too (#3449)

* tools: check-theme: consider `background-color` too

This is the precise property we want to check.

Author: felipec

tools/checkTheme.js

@@ -193,7 +193,10 @@ class CSSRule {
   constructor(rule, body) {
     this.rule = rule;
     if (rule.declarations) {
-      this.bg = rule.declarations.find(x => x.property =="background")?.value;
+      this.bg = rule.declarations.find(x => x.property == "background-color")?.value;
+      if (!this.bg) {
+        this.bg = rule.declarations.find(x => x.property == "background")?.value;
+      }
       this.fg = rule.declarations.find(x => x.property =="color")?.value;
 
       if (this.bg) {