So you made a change the look of your hub site using SharePoint Theme but the changes are taking too long to get pushed to your Associated site collection? 

There is one work around for this situation, you can simply re-apply your Hub site association by following these steps. 

Step 1: Navigate to your new SharePoint admin center

Go to your SharePoint Admin center and click Try it now to new SharePoint admin center

Select Active sites from your new SharePoint Admin center

Step 2: Re-assign the hub association 

Select the site where you want to get the updates immediately and from command bar -> Select change hub site association

Select None and click Save

Now the association has been removed, its time to apply it again. Select the site -> and from command bar -> Select change hub site association -> Set it back to what it was

That's it, now you can see your changes applied in Hub site!