Just out of curiosity, what version of Totara are you using with this Lamda theme?

Hi Hoa

Themes development vary between versions of Totara - see the theme documentation on ways themes can be developed.

If you are just wanting a minor modification such as increasing the logo size then Overwriting CSS is the simplest way to achieve this.

For example in Totara 10 the area for the logo is currently limited to 70px high. If the logo is taller than that it will be scaled to fit.

Either the height of the logo needs to be decreased to prevent it from scaling, or the theme needs to be customised to increase the height of the logo containers.
If you choose to customise I believe you will need to set an appropriate height using custom CSS in the Basis theme for example 

.navbar-site .navbar-logo {height: 172px;}

regards