doc: tweak API CSS for improved usability
doc: tweak API CSS for improved usability

API documentation (generated from doxygen comments) visibility
improvement by using a light-blue background on fields (vs. grey),
matching the Kconfig option documentation.

Signed-off-by: David B. Kinder <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,ldts/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,ldts/zephyr,finikorg/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,finikorg/zephyr,Vudentz/zephyr,explora26/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr