Modernism is the idea that there are no eternal truths, that truth is the correspondence of the mind with one's lifestyle (adaequatio intellectus et vitae), and that, therefore, old dogmas must be abandoned and new beliefs must arise that meet 'the needs of modern man'. This is a radical denial of the traditional and common sense notion of truth: the correspondence of the mind with reality (adaequatio intellectus et rei), which is the basis of the immutability of Catholic dogma.
No, the post-conciliar theological principle is neo-modernism, and the theology that is based on it is known as the nouvelle theologie. It is the idea that old dogmas or beliefs must be retained, yet not the traditional 'formulas': dogmas must be expressed and interpreted in a new way in every age so as to meet the 'needs of modern man'. This is still a denial of the traditional and common sense notion of truth as adaequatio intellectus et rei (insofar as it is still an attempt to make the terminology that expresses the faith correspond with our modern lifestyle) and consequently of the immutability of Catholic dogma, yet it is not as radical as modernism. It is more subtle and much more deceptive than modernism because it claims that the faith must be retained; it is only the 'formulas' of faith that must be abandoned--they use the term 'formula' to distinguish the supposedly mutable words of our creeds, dogmas, etc. from their admittedly immutable meanings. Therefore, neo-modernism can effectively slip under the radar of most pre-conciliar condemnations (except Humani generis, which condemns it directly) insofar as its practitioners claim that their new and unintelligible theological terminology really expresses the same faith of all times. In other words, neo-modernism is supposed to be 'dynamic orthodoxy': supposedly orthodox in meaning, yet always changing in expression to adapt to modern life (cf. Franciscan University of Steubenville's mission statement).
Take extra ecclesiam nulla salus as a clear example of a dogma that has received a brutal neo-modernist re-interpretation: they claim that the old 'formula' that "there is no salvation outside the Church" must be abandoned; rather it is more meaningful to modern man to say that salvation is not in, but through, the Church; people who are not in the Church may still be saved through the Church; thus, to them the dogma that "there is no salvation outside the Church" means that there is salvation outside the Church. Hence see Ven. Pope Pius XII condemning those "reduce to a meaningless formula the necessity of belonging to the true Church in order to gain eternal salvation." (Humani generis 27).