fix tabs title font size

[YiluSu]

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[afc163]

This one should be @font-size-base

[YiluSu]

would it make small tab have 14px font-size? as defined at top of this file:

@font-size-base         : 14px;
@font-size-lg           : @font-size-base + 2px;
@font-size-sm           : 12px;

I thought small, default and large tab are 12px, 14px and 16px, respectively.

[afc163]

Right, the font size of small tabs should be 14px.