Hi Hemant,

note: It is not recommended to change heights of toolbars / buttons or other elements, unless you are an expert with CSS, as this may break the responsive design that scales your forms nicely to mobile devices of various sizes.

To create a custom theme, you will need to copy an existing theme (with its associated folders, sub-folders and files), and then edit the copied theme as required. 

For more details you can check the link below under "Detailed steps"

https://help.k2.com/onlinehelp/K2Five/DevRef/5.3/default.htm#Themes/Custom-Themes.htm

Should you feel that this post is of use and or an accurate solution to the raised question, I kindly encourage you to mark it as such
using the 'Mark as Solution', 'Kudo' andor ‘Me Too’ options.

Cheers,
Kate

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