There are many programs about for creating web pages, and you all have your favorites. All the better platforms can accommodate the SendReach Optin Form HTML Code and provide you with the means to apply a style of your choice to it.
You can however add simple styling, with a choice of background and text colour, by adding the code downloaded with this link immediately above the Optin Form HTML code.
OptinFormSimpleStyleCodeDownload – copy and paste the code above your Optin Form code
The simple styling produces this style of form, in which the background colour can be changed in the code.