The short answer is ‘Yes, it can do’, but not necessarily always for the worse.
It could improve your SEO or it could make it worse, and the effect it has depends on a few things:
- How you set up your META tags (SEO title, SEO description, index/noindex tags, etc)
- How good the coding of the new theme is
- Whether the new theme has links hidden in the footer
- How quickly pages load with the new theme
So let’s take a closer look.
Basically, anything you’ve done with your existing theme, which would include customisations to the design (for example changing the font), and could include how you insert the META tags to your pages and posts, will be lost when you change themes.
Here’s a simple rule of thumb: any changes you make to your theme apply only to your theme, but any changes you make with plugins are applied to all themes.
An example: if you use one of the SEO plugins to add the SEO title and description, and control your index and follow tags, then whatever you set up in that plugin will be applied to your site, no matter what theme you use.
But if you use your theme to add those SEO factors then they will be lost when you change your theme.
Another example: apply the Google Analytics code via a field in your theme and all will be good until you change your theme. When you change your theme the Google Analytics code will disappear.
If you add the Google Analytics code via a plugin, then it doesn’t matter which theme you use – the Google Analytics code will always be in place.
Bad or bloated coding
The quality of the coding in themes has one of the biggest impacts in how quickly your site pages load, and page load speed is a big factor in how well your site does in the search results.
And, believe it or not, some themes do not handle Header tags (H tags) in accordance with the W3C standards – which would definitely have a negative impact on the SEO effectiveness of your site.
Structured data is another area that some themes don’t handle at all – which doesn’t matter if your SEO (including structured data) is handled by an SEO plugin. But if it’s handled by your theme it would be a big problem
Some themes include an array of plugins which you may or may not want to use and simply add code bloat to your site.
For example, many of the themes you get from ThemeForest include the Woo Commerce plugin by default. If you are not offering a shop on your website then you don’t need Woo Commerce which, because it is a large and complex plugin, can be a real drag on page load times.
And simply deactivating Woo Commerce is not always the answer.
I’ve seen themes where deactivating Woo Commerce actually affected the layout of non-shop pages – an example of an extremely poorly coded theme.
Slider plugins are also popular default additions in Themeforest themes. Slider plugins, at least in my view, add no value whatsoever to a site and negatively impact the SEO.
They add a whole bunch of code, bloating your page, and visitors either rarely see more than the first image because they’ve already scrolled down to read the content, or cannot read any of the images because they cycle through too quickly.
I’ve seen sites with image sliders that turned me off completely and caused me to click away without even attempting to read the content because they were so jarring.
If you want a slider, then one of those themes would be fine. But sliders have lost most of their popularity over the past few years and make a site look dated now.
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Hidden links in the footer
There are still plenty of themes you find online that contain links hidden in the footer, and a hidden link is generally a bad link.
They can be put there for any reason, but the only acceptable links in the footer are to analytics tracking sites (such as Statcounter) or ‘Honeypots’. Honeypots are there to lure spammers so that the originating email domain and IP address can be added to a blacklist.
Any other hidden link is bad and will not only negatively affect your SEO, it will get your website blacklisted.
So if your current theme is clean (no hidden links) and your new theme is not, then you may end up having your site blacklisted.
So to answer your question…
Yes, changing your theme can affect your SEO, but it could do so either positively or negatively.
So before moving forward with a change like that carefully consider what you might lose because it has been coded into your current theme.
And when looking for a new theme always remember that you get what you pay for. A well-coded theme that loads quickly, has no hidden links, doesn’t cause any of your META tags to be lost, handles all your H tags correctly and is coded to W3C standards could well improve your SEO.
It may well cost more than that US$29 theme from ThemeForest, but the extra price will be well worth it.