Since SCCM 1710 there was the addition to device settings to allow customisation of software center 1806 added an additional customisation to add a custom web page as an additional tab. As above you need to be at 1710 or above to have these customisation options.
by default the below would be the basic colour and appearance for the software center:
To start I usually create a new policy to test before pushing out to all users.
- Open SCCM console
- Go to Administration > Client settings
Right click and create a new policy (I called mine Custom Software Center)
Click the Software Center check box in the policy
Under device settings set select these new settings to specify company information to yes. 
Then go to customize. Edit company name, set color scheme and add a company picture. Below is the default settings. The logo image file needs to be a maximum size is 400×100 and 750kb
Here is the updated settings
To add a custom tab with URL to a support site or blog post. Go to tabs and put a check box on: Specify a custom tab for Software Center. (This is only available if SCCM is 1806 or above.)
- Add a custom tab name
- Add a URL
Once finished editing the policy it then needs to be deployed to the require collection. Right click on the custom policy and go to deploy.
Then select the device collection to deploy to
Once the policy is deployed, go to a device in the collection to update the machine policy. Go to Control Panel > Configuration Manager > Actions > Machine policy retrieval & Evaluation cycle and click run now. 
Once the policy is updated the Software Center should be updated with the new color scheme, company logo and custom URL
TheSleepyAdmins 