correct CSS validity errors (#2151)
correct CSS validity errors (#2151)

@suztomo 

per https://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fjlbp.dev&profile=css3svg&usermedium=all&warning=1&vextwarning=&lang=en

part of #354
GoogleCloudPlatform/cloud-opensource-java,GoogleCloudPlatform/cloud-opensource-java,GoogleCloudPlatform/cloud-opensource-java