You can change the CSS for each Workflow Form. This can be done by clicking on Custom Styles button on Form Design window:

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There you can find default CSS and customize it. You can also select file as CSS source instead of providing it in Workflow Designer and also restore default CSS settings.

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Changing default CSS file

You can change default CSS file, so you do not need to change CSS settings for each Workflow Form.

Follow the below PowerShell script example (the setting is web-scoped):

Add-PSSnapin "Microsoft.SharePoint.PowerShell"

$dpca = [Reflection.Assembly]::Load("Datapolis.WorkBox.Common, Version=1.1.0.0, Culture=neutral, PublicKeyToken=2c03c3097ac016b8")

$webUrl = "http://gasco-dev-ks/sites/DPSPL"

$newCssUrl = "/_layouts/15/Datapolis.WorkBox/WBActionForm_Custom.css"

$web = Get-SPWeb $webUrl

[Datapolis.WorkBox.Common.PowerShell.Configuration]::SetDefaultActionFormCss($web, $newCssUrl)

Default css file is "/_layouts/15/Datapolis.WorkBox/WBActionForm.css":