frontend: Recognize OBS appearance setting usage in math statements

The check for whether an OBS appearance setting is used in a theme only
checks whether there is a direct alias to an injected variable, like
`--font_base_value: var(--obsFontScale)`. Math statements like
`--font_base_value: calc(1 + var(--obsFontScale))` would not get
recognized, so if a theme only uses such statements (no direct aliases),
the check would wrongfully conclude that the appearance setting is not
used, which would later lead to the setting being disabled in the UI.
The value however would be injected correctly.

This can be mitigated by checking whether the arguments of the math
operations contain the expression.

frontend/OBSApp_Themes.cpp

@@ -880,16 +880,32 @@ bool OBSApp::SetTheme(const QString &name)
 	/* Check if OBS appearance settings are used in the theme */
 	currentTheme->usesFontScale = false;
 	currentTheme->usesDensity = false;
-	for (const OBSThemeVariable &var_ : vars) {
-		if (var_.type != OBSThemeVariable::Alias)
+	for (const OBSThemeVariable &var : vars) {
+		switch (var.type) {
+		case OBSThemeVariable::Color:
+		case OBSThemeVariable::Size:
+		case OBSThemeVariable::Number:
+		case OBSThemeVariable::String:
 			continue;
+		case OBSThemeVariable::Alias:
+			if (var.value.toString() == "obsFontScale") {
+				currentTheme->usesFontScale = true;
+			}
 
-		if (var_.value.toString() == "obsFontScale") {
-			currentTheme->usesFontScale = true;
-		}
-
-		if (var_.value.toString() == "obsPadding") {
-			currentTheme->usesDensity = true;
+			if (var.value.toString() == "obsPadding") {
+				currentTheme->usesDensity = true;
+			}
+			break;
+		case OBSThemeVariable::Calc:
+		case OBSThemeVariable::Max:
+		case OBSThemeVariable::Min:
+			if (var.value.toStringList().contains("obsFontScale")) {
+				currentTheme->usesFontScale = true;
+			}
+			if (var.value.toStringList().contains("obsPadding")) {
+				currentTheme->usesDensity = true;
+			}
+			break;
 		}
 	}