fix low contrast on night theme embed frame "Export as pdf" (#46288)

* update embed frame pdf button colors to use --mb-color- vars

* change download pdf button color to brand color

* testing adding the images that actually differ

frontend/src/metabase/core/components/TabButton/TabButton.styled.tsx

@@ -51,7 +51,9 @@ export const TabButtonRoot = styled.div<TabButtonProps>`
   height: 1.875rem;
   padding: 0.25rem;
   color: ${props =>
-    props.isSelected && !props.disabled ? color("brand") : color("text-dark")};
+    props.isSelected && !props.disabled
+      ? "var(--mb-color-brand)"
+      : "var(--mb-color-text-primary)"};
   opacity: ${props => (props.disabled ? 0.3 : 1)};
   font-size: 0.75rem;
   font-weight: 700;

frontend/src/metabase/public/components/EmbedFrame/EmbedFrame.tsx

@@ -209,7 +209,7 @@ export const EmbedFrame = ({
                     <Button
                       variant="subtle"
                       leftIcon={<Icon name="document" />}
-                      color="text-dark"
+                      color="brand"
                       onClick={saveAsPDF}
                     >
                       {getExportTabAsPdfButtonText(dashboard.tabs)}